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Arithmetic
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2.1 Arithmetic
 
2.2 Algebra
 
 
2.3 Geometry
 
2.4 Data Analysis
 
Arithmetic
Arithmetic is the fundamental building block of math. The other three subject areas tested in GRE Math are all pretty much unthinkable without arithmetic. You’ll certainly need to know your arithmetic to power through algebra, geometry, and data analysis problems, but the Math section also includes some pure arithmetic problems as well. So it makes sense to start Math 101 with a discussion of numbers and the typical things we do with them.
Common Math Symbols
You may remember these from way back when, but in case you need a quick refresher, here’s a list of some of the most commonly used math symbols you should know for the GRE. We’ll discuss some of them in this arithmetic section and others later in the chapter.
Symbol Name Meaning
< Less than The quantity to the left of the symbol is less than the quantity to the right.
> Greater than The quantity to the left of the symbol is greater than the quantity to the right.
≤ Less than or equal to The quantity to the left of the symbol is less than or equal to the quantity to the right.
≥ Greater than or equal to The quantity to the left of the symbol is greater than or equal to the quantity to the right.
Square root A number which when multiplied by itself equals the value under the square root symbol.
| x | Absolute value The positive distance a number enclosed between two vertical bars is from 0.
! Factorial The product of all the numbers up to and including a given number.
|| Parallel In geometry, two lines separated by this symbol have the same slope (go in exactly the same direction).
Perpendicular In geometry, two lines separated by this symbol meet at right angles.
° Degrees A measure of the size of an angle. There are 360 degrees in a circle.
π Pi The ratio of the circumference of any circle to its diameter; approximately equal to 3.14.
Number Terms
The test makers assume that you know your numbers. Make sure you do by comparing your knowledge to our definitions below.
Number Definition Example
Whole numbers The set of counting numbers, including zero 0, 1, 2, 3
Natural numbers The set of whole positive numbers except zero 1, 2, 3, 4
Integers The set of all positive and negative whole numbers, including zero, not including fractions and decimals. Integers in a sequence, such as those in the example to the right, are called consecutive integers. –3, –2, –1, 0, 1, 2, 3
Rational numbers The set of all numbers that can be expressed as integers in fractions—that is, any number that can be expressed in the form , where m and n are integers
Irrational numbers The set of all numbers that cannot be expressed as integers in a fraction π, , 1.010100001000110000
Real numbers Every number on the number line, including all rational and irrational numbers Every number you can think of
Even and Odd Numbers
An even number is an integer that is divisible by 2 with no remainder, including zero.
Even numbers: –10, –4, 0, 4, 10
An odd number is an integer that leaves a remainder of 1 when divided by 2.
Odd numbers: –9, –3, –1, 1, 3, 9
Even and odd numbers act differently when they are added, subtracted, multiplied, and divided. The following chart shows the rules for addition, subtraction, and multiplication (multiplication and division are the same in terms of even and odd).
Addition Subtraction Multiplication/Division
even + even = even even – even = even even × even = even
even + odd = odd even – odd = odd even × odd = even
odd + odd = even odd – odd = even odd × odd = odd
Zero, as we’ve mentioned, is even, but it has its own special properties when used in calculations. Anything multiplied by 0 is 0, and 0 divided by anything is 0. However, anything divided by 0 is undefined, so you won’t see that on the GRE.
Positive and Negative Numbers
A positive number is greater than 0. Examples include , 15, and 83.4. A negative number is less than 0. Examples include –0.2, –1, and –100. One tip-off is the negative sign (–) that precedes negative numbers. Zero is neither positive nor negative. On a number line, positive numbers appear to the right of zero, and negative numbers appear to the left:
–5, –4, –3, –2, –1, 0, 1, 2, 3, 4, 5
Positive and negative numbers act differently when you add, subtract, multiply, or divide them. Adding a negative number is the same as subtracting a positive number:
5 + (–3) = 2, just as 5 – 3 = 2
Subtracting a negative number is the same as adding a positive number:
7 – (–2) = 9, just as 7 + 2 = 9
To determine the sign of a number that results from multiplication or division of positive and negative numbers, memorize the following rules.
Multiplication Division
positive × positive = positive positive ÷ positive = positive
positive × negative = negative positive ÷ negative = negative
negative × negative = positive negative ÷ negative = positive
Here’s a helpful trick when dealing with a series of multiplied or divided positive and negative numbers: If there’s an even number of negative numbers in the series, the outcome will be positive. If there’s an odd number, the outcome will be negative.
When negative signs and parentheses collide, it can get pretty ugly. However, the principle is simple: A negative sign outside parentheses is distributed across the parentheses. Take this question:
3 + 4 – (3 + 1 – 8) = ?
You’ll see a little later on when we discuss order of operations that in complex equations we first work out the parentheses, which gives us:
3 + 4 – (4 – 8)
This can be simplified to:
3 + 4 – (– 4)
As discussed earlier, subtracting a negative number is the same as adding a positive number, so our equation further simplifies to:
3 + 4 + 4 = 11
An awareness of the properties of positive and negative numbers is particularly helpful when comparing values in Quantitative Comparison questions, as you’ll see later in chapter 4.
Remainders
A remainder is the integer left over after one number has been divided by another. Take, for example, 92 ÷ 6. Performing the division we see that 6 goes into 92 a total of 15 times, but 6 × 15 = 90, so there’s 2 left over. In other words, the remainder is 2.
Divisibility
Integer x is said to be divisible by integer y when x divided by y yields a remainder of zero. The GRE sometimes tests whether you can determine if one number is divisible by another. You could take the time to do the division by hand to see if the result is a whole number, or you could simply memorize the shortcuts in the table below. Your choice. We recommend the table.
Divisibility Rules
1 All whole numbers are divisible by 1.
2 A number is divisible by 2 if it’s even.
3 A number is divisible by 3 if the sum of its digits is divisible by 3. This means you add up all the digits of the original number. If that total is divisible by 3, then so is the number. For example, to see whether 83,503 is divisible by 3, we calculate 8 + 3 + 5 + 0 + 3 = 19. 19 is not divisible by 3, so neither is 83,503.
4 A number is divisible by 4 if its last two digits, taken as a single number, are divisible by 4. For example, 179,316 is divisible by 4 because 16 is divisible by 4.
5 A number is divisible by 5 if its last digit is 0 or 5. Examples include 0, 430, and –20.
6 A number is divisible by 6 if it’s divisible by both 2 and 3. For example, 663 is not divisible by 6 because it’s not divisible by 2. But 570 is divisible by 6 because it’s divisible by both 2 and 3 (5 + 7 + 0 = 12, and 12 is divisible by 3).
7 7 may be a lucky number in general, but it’s unlucky when it comes to divisibility. Although a divisibility rule for 7 does exist, it’s much harder than dividing the original number by 7 and seeing if the result is an integer. So if the GRE happens to throw a “divisible by 7” question at you, you’ll just have to suck it up and do the math.
8 A number is divisible by 8 if its last three digits, taken as a single number, are divisible by 8. For example, 179,128 is divisible by 8 because 128 is divisible by 8.
9 A number is divisible by 9 if the sum of its digits is divisible by 9. This means you add up all the digits of the original number. If that total is divisible by 9, then so is the number. For example, to see whether 531 is divisible by 9, we calculate 5 + 3 + 1 = 9. Since 9 is divisible by 9, 531 is as well.
10 A number is divisible by 10 if the units digit is a 0. For example, 0, 490, and –20 are all divisible by 10.
11 This one’s a bit involved but worth knowing. (Even if it doesn’t come up on the test, you can still impress your friends at parties.) Here’s how to tell if a number is divisible by 11: Add every other digit starting with the leftmost digit and write their sum. Then add all the numbers that you didn’t add in the first step and write their sum. If the difference between the two sums is divisible by 11, then so is the original number. For example, to test whether 803,715 is divisible by 11, we first add 8 + 3 + 1 = 12. To do this, we just started with the leftmost digit and added alternating digits. Now we add the numbers that we didn’t add in the first step: 0 + 7 + 5 = 12. Finally, we take the difference between these two sums: 12 – 12 = 0. Zero is divisible by all numbers, including 11, so 803,715 is divisible by 11.
12 A number is divisible by 12 if it’s divisible by both 3 and 4. For example, 663 is not divisible by 12 because it’s not divisible by 4. 162,480 is divisible by 12 because it’s divisible by both 4 (the last two digits, 80, are divisible by 4) and 3 (1 + 6 + 2 + 4 + 8 + 0 = 21, and 21 is divisible by 3).
Factors
A factor is an integer that divides into another integer evenly, with no remainder. In other words, if is an integer, then b is a factor of a. For example, 1, 2, 4, 7, 14, and 28 are all factors of 28, because they go into 28 without having anything left over. Likewise, 3 is not a factor of 28 since dividing 28 by 3 yields a remainder of 1. The number 1 is a factor of every number.
Some GRE problems may require you to determine the factors of a number. To do this, write down all the factors of the given number in pairs, beginning with 1 and the number you’re factoring. For example, to factor 24:
  • 1 and 24 (1 × 24 = 24)
  • 2 and 12 (2 × 12 = 24)
  • 3 and 8 (3 × 8 = 24)
  • 4 and 6 (4 × 6 = 24)
Five doesn’t go into 24, so you’d move on to 6. But we’ve already included 6 as part of the 4 × 6 equation, and there’s no need to repeat. If you find yourself beginning to repeat numbers, then the factorization’s complete. The factors of 24 are therefore 1, 2, 3, 4, 6, 8, 12, and 24.
Prime Numbers
Everyone’s always insisting on how unique they are. Punks wear leather. Goths wear black. But prime numbers actually are unique. They are the only numbers whose sole factors are 1 and themselves. More precisely, a prime number is a number that has exactly two positive factors, 1 and itself. For example, 3, 5, and 13 are all prime, because each is only divisible by 1 and itself. In contrast, 6 is not prime, because, in addition to being divisible by 1 and itself, 6 is also divisible by 2 and 3. Here are a couple of points about primes that are worth memorizing:
  • All prime numbers are positive. This is because every negative number has –1 as a factor in addition to 1 and itself.
  • The number 1 is not prime. Prime numbers must have two positive factors, and 1 has only one positive factor, itself.
  • The number 2 is prime. It is the only even prime number. All prime numbers besides 2 are odd.
Here’s a list of the prime numbers less than 100:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, and 97
It wouldn’t hurt to memorize this list. In addition, you can determine whether a number is prime by using the divisibility rules listed earlier. If the number is divisible by anything other than 1 and itself, it’s not prime.
If a number under consideration is larger than the ones in the list above, or if you’ve gone and ignored our advice to memorize that list, here’s a quick way to figure out whether a number is prime:
  1. Estimate the square root of the number.
  2. Check all the prime numbers that fall below your estimate to see if they are factors of the number. If no prime below your estimate is a factor of the number, then the number is prime.
Let’s see how this works using the number 97.
  1. Estimate the square root of the number:
  2. Check all the prime numbers that fall below 10 to see if they are factors of 97: Is 97 divisible by 2? No, it does not end with an even number. Is 97 divisible by 2? No, it does not end with an even number. Is 97 divisible by 3? No, 9 + 7 = 16, and 16 is not divisible by 3. Is 97 divisible by 5? No, 97 does not end with 0 or 5. Is 97 divisible by 7? No, 97 ÷ 7 = 13, with a remainder of 6.
Therefore, 97 is prime. (Of course, you knew that already from familiarizing yourself with the prime numbers less than 100. . . .)
Prime Factorization
Come on, say it aloud with us: “prime factorization.” Now imagine Arnold Schwarzenegger saying it. Then imagine if he knew how to do it. Holy Moly. He would probably be governor of the entire United States!
A math problem may ask you to directly calculate the prime factorization of a number. Other problems, such as those involving greatest common factors or least common multiples (which we’ll discuss soon), are easier to solve if you know how to calculate the prime factorization. Either way, it’s good to know how to do it.
To find the prime factorization of a number, divide it and all its factors until every remaining integer is prime. The resulting group of prime numbers is the prime factorization of the original integer. Want to find the prime factorization of 36? We thought so:
36 = 2 × 18 = 2 × 2 × 9 = 2 × 2 × 3 × 3
That’s two prime 2s, and two prime 3s, for those of you keeping track at home.
It can be helpful to think of prime factorization in the form of a tree:
As you may already have noticed, there’s more than one way to find the prime factorization of a number. Instead of cutting 36 into 2 and 18, you could have factored it into 6 × 6, and then continued from there. As long as you don’t screw up the math, there’s no wrong path—you’ll always get the same result.
Let’s try one more example. The prime factorization of 220 could be found like so:
220 = 10 × 22
10 is not prime, so we replace it with 5 × 2:
10 × 22 = 2 × 5 × 22
22 is not prime, so we replace it with 2 × 11:
2 × 5 × 22 = 2 × 2 × 5 × 11
2, 5, and 11 are all prime, so we’re done. The prime factorization of 220 is thus 2 × 2 × 5 × 11.
Greatest Common Factor
The greatest common factor (GCF) of two numbers is the largest number that is a factor of both numbers—that is, the GCF is the largest factor that both numbers have in common. For example, the GCF of 12 and 18 is 6, because 6 is the largest number that divides evenly into 12 and 18. Put another way, 6 is the largest number that is a factor of both 12 and 18.
To find the GCF of two numbers, you can use their prime factorizations. The GCF is the product of all the numbers that appear in both prime factorizations. In other words, the GCF is the overlap of the two factorizations.
For example, let’s calculate the GCF of 24 and 150. First, we figure out their prime factorizations:
24 = 2 × 2 × 2 × 3
150 = 2 × 3 × 5 × 5
Both factorizations contain 2 × 3. The overlap of the two factorizations is 2 and 3. The product of the overlap is the GCF. Therefore, the GCF of 24 and 150 is 2 × 3 = 6.
Multiples
A multiple can be thought of as the opposite of a factor: If is an integer, then x is a multiple of y. Less formally, a multiple is what you get when you multiply an integer by another integer. For example, 7, 14, 21, 28, 70, and 700 are all multiples of 7, because they each result from multiplying 7 by an integer. Similarly, the numbers 12, 20, and 96 are all multiples of 4 because 12 = 4 × 3, 20 = 4 × 5, and 96 = 4 × 24. Keep in mind that zero is a multiple of every number. Also, note that any integer, n, is a multiple of 1 and n, because 1 × n = n.
Least Common Multiple
The least common multiple (LCM) of two integers is the smallest number that is divisible by the two original integers. As with the GCF, you can use prime factorization as a shortcut to find the LCM. For example, to find the least common multiple of 10 and 15, we begin with their prime factorizations:
10 = 5 × 2
15 = 5 × 3
The LCM is equal to the product of each factor by the maximum number of times it appears in either number. Since 5 appears once in both factorizations, we need to include it once in our final product. The same goes for the 2 and the 3, since each of these numbers appears one time in each factorization. The LCM of 10 and 15, then, is 5 × 3 × 2 = 30. In other words, 30 is the smallest number that is divisible by both 10 and 15. Remember that the LCM is the least common multiple—you have to choose the smallest number that is a multiple of each original number. So, even though 60 is a multiple of both 10 and 15, 60 is not the LCM, because it’s not the smallest multiple of those two numbers.
This is a bit tricky, so let’s try it again with two more numbers. What’s the LCM of 60 and 100?
First, find the prime factorizations:
60 = 2 × 2 × 3 × 5
100 = 2 × 2 × 5 × 5
So, 2 occurs twice in each of these factorizations, so we’ll need to include two 2s in our final product. We have one 5 in our factorization of 60, but two 5s in our factorization of 100. Since we’re looking to include the maximum number of appearances of each factor, we’ll include two 5s in our product. There’s also one 3 in the first factorization, and no 3s in the second, so we have to add one 3 to the mix. This results in an LCM of 2 × 2 × 3 × 5 × 5 = 300.
Order of Operations
What if you see something like this on the test:
You basically have two choices. You can (a) run screaming from the testing site yelling “I’ll never, ever, EVER get into graduate school!!!” or (b) use PEMDAS.
PEMDAS is an acronym for the order in which mathematical operations should be performed as you move from left to right through an expression or equation. It stands for:
  • Parentheses
  • Exponents
  • Multiplication
  • Division
  • Addition
  • Subtraction
You may have had PEMDAS introduced to you as “Please Excuse My Dear Aunt Sally.” Excuse us, but that’s a supremely lame 1950s-style acronym. We prefer, Picking Eminem Made Dre A Star. Whatever. Come up with one of your own if you want. Just remember PEMDAS.
If an equation contains any or all of these PEMDAS elements, first carry out the math within the parentheses, then work out the exponents, then the multiplication, and the division. Addition and subtraction are actually a bit more complicated. When you have an equation to the point that it only contains addition and subtraction, perform each operation moving from left to right across the equation. Let’s see how this all plays out in the context of the example above:
First work out the math in the parentheses, following PEMDAS even within the parentheses. So here we focus on the second parentheses and do the multiplication before the subtraction:
Now taking care of the subtraction in both sets of parentheses:
Now work out the exponent (more on those later):
Then do the multiplication:
Then the division:
12­­­ – 7 + 17
We’re left with just addition and subtraction, so we simply work from left to right:
5 + 17
And finally:
22
Piece of cake! Well, not exactly, but it beats fleeing the room in hysterics. PEMDAS is the way to crunch down the most difficult-looking equations or expressions. Take it one step at a time, and you’ll do just fine.
Fractions
Much of what we’ve covered so far concerns whole numbers. Now we enter the vast universe that exists between those nice round numbers: the world of fractions. The GRE loves fractions. The number of questions on the Math section that involve fractions in some way or another is nothing short of stupefying. This means you must know fractions inside and out. Know how to compare them, reduce them, add them, and multiply them. Know how to divide them, subtract them, and convert them to mixed numbers. Know them. Love them like the GRE does. Make them your friend on the test, not your enemy.
To begin, here are the basics: A fraction is a part of a whole. It’s composed of two expressions, a numerator and a denominator. The numerator of a fraction is the quantity above the fraction bar, and the denominator is the quantity below the fraction bar. For example, in the fraction , 1 is the numerator and
2 is the denominator. The denominator tells us how many units there are in all, while the numerator tells us how many units out of that total are specified in a given instance. For example, if your friend has five cookies and offers you two of them, you’d be entitled to eat of her cookies. How many you sneak when she’s not looking is up to you.
The general concept of fractions isn’t difficult, but things can get dicey when you have to do things with them. Hence, the following subtopics that you need to have under your belt.
Fraction Equivalencies
Fractions represent a part of a whole, so if you increase both the part and whole by the same multiple, you will not change the relationship between the part and the whole.
To determine if two fractions are equivalent, multiply the denominator and numerator of one fraction so that the denominators of the two fractions are equal (this is one place where knowing how to calculate LCM and GCF comes in handy). For example, because if you multiply the numerator and denominator of by 3, you get:. As long as you multiply or divide both the numerator and denominator of a fraction by the same nonzero number, you will not change the overall value of the fraction.
Reducing Fractions
Reducing fractions makes life simpler, and we all know life is complicated enough without crazy fractions weighing us down. Reducing takes unwieldy monsters like and makes them into smaller, friendlier critters. To reduce a fraction to its lowest terms, divide the numerator and denominator by their GCF. For example, for , the GCF of 450 and 600 is 150. So the fraction reduces down to , since and.
A fraction is in its simplest, totally reduced form when the GCF of its numerator and denominator is 1. There is no number but 1, for instance, that can divide into both 3 and 4, so is a fraction in its lowest form, reduced as far as it can go. The same goes for the fraction , but is a different story because 3 is a common factor of both the numerator and denominator. Dividing each by this common factor yields , the fraction in its most reduced form.
Adding, Subtracting, and Comparing Fractions
To add fractions with the same denominators, all you have to do is add up the numerators and keep the denominator the same:
Subtraction works similarly. If the denominators of the fractions are equal, just subtract one numerator from the other and keep the denominator the same:
Remember that fractions can be negative too:
Some questions require you to compare fractions. Again, this is relatively straightforward when the denominators are the same. The fraction with the greater numerator will be the larger fraction. For example, is greater than , while is greater than . (Be careful of those negative numbers! Since –5 is less negative than –13, –5 is greater than –13.)
Working with fractions with the same denominators is one thing, but working with fractions with different denominators is quite another. So we came up with an easy alternative: the Magic X. For adding, subtracting, and comparing fractions with different denominators, the Magic X is a lifesaver. Sure, you can go ahead and find the least common denominator, a typical way of tackling such problems, but we don’t call our trick the “Magic X” for nothing. Here’s how it works in each situation.
Adding. Consider the following equation:
You could try to find the common denominator by multiplying by 9 and by 7, but then you’d be working with some pretty big numbers. Keep things simple, and use the Magic X. The key is to multiply diagonally and up, which in this case means from the 9 to the 3 and also from the 7 to the 2:
In an addition problem, we add the products to get our numerator: 27 + 14 = 41. For the denominator, we simply multiply the two denominators to get:
Believe it or not, we’re already done! The numerator is 41, and the denominator is 63, which results in a final answer of .
Subtracting. Same basic deal, except this time we subtract the products that we get when we multiply diagonally and up. See if you can feel the magic in this one:
Multiplying diagonally and up gives:
The problem asks us to subtract fractions, so this means we need to subtract these numbers to get our numerator: 24 – 25 = –1. Just like in the case of addition, we multiply across the denominators to get the denominator of our answer:
That’s it! The numerator is –1 and the denominator is 30, giving us an answer of . Not the prettiest number you’ll ever see, but it’ll do.
Comparing. The Magic X is so magical that it can also be used to compare two fractions, with just a slight modification: omitting the step where we multiply the denominators. Say you’re given the following Quantitative Comparison problem. We’ll explain much more about QCs in chapter 4, but for now remember that the basic idea is to compare the quantity in Column A with the quantity in Column B to see which, if either, is bigger. (In some cases, the answer will be that you can’t determine which is bigger, but as you’ll learn, when the two quantities are pure numbers with no variables, that option is impossible.) See what you can make of this sample QC:
Column A Column B

Now, if you were a mere mortal with no magic at your fingertips, this would be quite a drag. But the Magic X makes it a pleasure. Again, begin by multiplying diagonally and up:
Now compare the numbers you get: 161 is larger than 150, so is greater than . Done.
Why does this work? Who knows? Who cares? It just does. (Actually, the rationale isn’t too complex, but it doesn’t add anything to your GRE repertoire, so let’s skip it.) Learn how to employ the Magic X in these three circumstances, and you’re likely to save yourself some time and effort.
Multiplying Fractions
Multiplying fractions is a breeze, whether the denominators are equal or not. The product of two fractions is merely the product of their numerators over the product of their denominators:
Want an example with numbers? You got one:
Canceling Out. You can make multiplying fractions even easier by canceling out. If the numerator and denominator of any of the fractions you need to multiply share a common factor, you can divide by the common factor to reduce both numerator and denominator before multiplying. For example, consider this fraction multiplication problem:
You could simply multiply the numerators and denominators and then reduce, but that would take some time. Canceling out provides a shortcut. We can cancel out the numerator 4 with the denominator 8 and the numerator 10 with the denominator 5, like this:
Then, canceling the 2s, you get:
Canceling out can dramatically cut the amount of time you need to spend working with big numbers. When dealing with fractions, whether they’re filled with numbers or variables, always be on the lookout for chances to cancel out.
Dividing Fractions
Multiplication and division are inverse operations. It makes sense, then, that to perform division with fractions, all you have to do is flip the second fraction and then multiply. Check it out:
Here’s a numerical example:
Compound Fractions. Compound fractions are nothing more than division problems in disguise. Here’s an example of a compound fraction:
It looks intimidating, sure, but it’s really only another way of writing , which now looks just like the previous example. Again, the rule is to invert and multiply. Take whichever fraction appears on the bottom of the compound fraction, or whichever fraction appears second if they’re written in a single line, and flip it over. Then multiply by the other fraction. In this case, we get . Now we can use our trusty canceling technique to reduce this to , or plain old 6. A far cry from the original!
Mixed Numbers
Sick of fractions yet? We don’t blame you. But there’s one topic left to cover, and it concerns fractions mixed with integers. Specifically, a mixed number is an integer followed by a fraction, like . But operations such as addition, subtraction, multiplication, and division can’t be performed on mixed numbers, so you have to know how to convert them into standard fraction form.
Since we already mentioned , it seems only right to convert it.
The method is easy: Multiply the integer (the 1) of the mixed number by the denominator of the fraction part, and add that product to the numerator: 1 × 3 + 2 = 5. This will be the numerator. Now, put that over the original denominator, 3, to finalize the converted fraction: .
Let’s try a more complicated example:
Pretty ugly as far as fractions go, but definitely something we can work with.
Decimals
A decimal is any number with a nonzero digit to the right of the decimal point. Like fractions, decimals are a way of writing parts of wholes. Some GRE questions ask you to identify specific digits in a decimal, so you need to know the names of these different digits. In this case, a picture is worth a thousand (that is, 1000.00) words:
Notice that all of the digits to the right of the decimal point have a th in their names.
In the number 839.401, for example, here are the values of the different digits.
Left of the decimal point Right of the decimal point
Units: 9 Tenths: 4
Tens: 3 Hundredths: 0
Hundreds: 8 Thousandths: 1
Converting Fractions to Decimals
So, what if a problem contains fractions, but the answer choices are all decimals? In that case, you’ll have to convert whatever fractional answer you get to a decimal. A fraction is really just shorthand for division. For example, is exactly the same as 6 ÷ 15. Dividing this out on your scratch paper results in its decimal equivalent, .4.
Converting Decimals to Fractions
What comes around goes around. If we can convert fractions to decimals, it stands to reason that we can also convert decimals to fractions. Here’s how:
  1. Remove the decimal point and make the decimal number the numerator.
  2. Let the denominator be the number 1 followed by as many zeros as there are decimal places in the original decimal number.
  3. Reduce this fraction if possible.
Let’s see this in action. To convert .3875 into a fraction, first eliminate the decimal point and place 3875 as the numerator:
Since .3875 has four digits after the decimal point, put four zeros in the denominator following the number 1:
We can reduce this fraction by dividing the numerator and denominator by the GCF, which is 125, or, if it’s too difficult to find the GCF right off the bat, we can divide the numerator and denominator by common factors such as 5 until no more reduction is possible. Either way, our final answer in reduced form comes out to .
Ratios
Ratios look like fractions and are related to fractions, but they don’t quack like fractions. Whereas a fraction describes a part of a whole, a ratio compares one part to another part.
A ratio can be written in a variety of ways. Mathematically, it can appear as or as 3:1. In words, it would be written out as “the ratio of 3 to 1.” Each of these three forms of the ratio 3:1 means the same thing: that there are three of one thing for every one of another. For example, if you have three red alligators and one blue alligator, then your ratio of red alligators to blue alligators would be 3:1. For the GRE, you must remember that ratios compare parts to parts rather than parts to a whole. Why do you have to remember that? Because of questions like this:
For every 40 games a baseball team plays, it loses 12 games. What is the ratio of the team’s losses to wins?
(A) 3:10
(B) 7:10
(C) 3:7
(D) 7:3
(E) 10:3
The question says that the team loses 12 of every 40 games, but it asks you for the ratio of losses to wins, not losses to games. So the first thing you have to do is find out how many games the team wins per 40 games played: 40 – 12 = 28. So for every 12 losses, the team wins 28 games, for a ratio of 12:28. You can reduce this ratio by dividing both sides by 4 to get 3 losses for every 7 wins, or 3:7. Choice C is therefore correct. If you instead calculated the ratio of losses to games played (part to whole), you might have just reduced the ratio 12:40 to 3:10, and then selected choice A. For good measure, the test makers include 10:3 to entice anyone who went with 40:12 before reducing. There’s little doubt that on ratio problems, you’ll see an incorrect part : whole choice and possibly these other kinds of traps that try to trip you up.
Proportions
Just because you have a ratio of three red alligators to one blue alligator doesn’t mean that you can only have three red alligators and one blue one. It could also mean that you have six red and two blue alligators or that you have 240 red and 80 blue alligators. (Not that we have any idea where you’d keep all those beasts, but you get the point.) Ratios compare only relative magnitude. To know how many of each color alligator you actually have, in addition to knowing the ratio, you also need to know how many total alligators there are. This concept forms the basis of another kind of ratio problem you may see on the GRE, a problem that provides you with the ratio among items and the total number of items, and then asks you to determine the number of one particular item in the group. Sounds confusing, but as always, an example should clear things up:

Egbert has red, blue, and green marbles in the ratio of 5:4:3, and he has a total of 36 marbles. How many blue marbles does Egbert have?

First let’s clarify what this means. For each group of 5 red marbles, Egbert (who does sound like a marble collector, doesn’t he?) has a group of 4 blue marbles and a group of 3 green marbles. If he has one group of each, then he’d simply have 5 red, 4 blue, and 3 green marbles for a total of 12. But he doesn’t have 12—we’re told he has 36. The key to this kind of problem is determining how many groups of each item must be included to reach the total. We have to multiply the total we’d get from having one group of each item by a certain factor that would give us the total given in the problem. Here, as we just saw, having one group of each color marble would give Egbert 12 marbles total, but since he has 36 marbles, we have to multiply by a factor of 3 (since 36 ÷ 12 = 3). That means Egbert has 3 groups of red marbles with 5 marbles in each group, for a total of 3 × 5 = 15 red marbles. Multiplying the other marbles by our factor of 3 gives us 3 × 4 = 12 blue marbles, and 3 × 3 = 9 green marbles. Notice that the numbers work out, because 15 + 12 + 9 does add up to 36 marbles total. The answer to the question is therefore 12 blue marbles.
So here’s the general approach: Add up the numbers given in the ratio. Divide the total items given by this number to get the factor by which you need to multiply each group. Then find the item type you’re looking for and multiply its ratio number by the factor you determined. In the example above, that would look like this:
5 (red) + 4 (blue) + 3 (green) = 12
36 ÷ 12 = 3 (factor)
4 (blue ratio #) × 3 (factor) = 12 (answer)
For the algebraic-minded among you, you can also let x equal the factor, and work the problem out this way:
5x + 4x + 3x = 36
12x = 36
x = 3
blue = (4)(3) = 12
Percents
Percents occur frequently in Data Interpretation questions but are also known to appear in Problem Solving and Quantitative Comparison questions as well. The basic concept behind percents is pretty simple: Percent means divide by 100. This is true whether you see the word percent or you see the percentage symbol, %. For example, 45% is the same as or .45.
Here’s one way percent may be tested:
4 is what percent of 20?
The first thing you have to know how to do is translate the question into an equation. It’s actually pretty straightforward as long as you see that “is” is the same as “equals,” and “what” is the same as “x.” So we can rewrite the problem as 4 equals x percent of 20, or:
Since a percent is actually a number out of 100, this means:
Now just work out the math:
Therefore, 4 is 20% of 20.
Percent problems can get tricky, because some seem to be phrased as if the person who wrote them doesn’t speak English. The GRE test makers do this purposefully because they think that verbal tricks are a good way to test your math skills. And who knows—they may even be right. Here’s an example of the kind of linguistic trickery we’re talking about:
What percent of 2 is 5?
Because the 2 is the smaller number and because it appears first in the question, your first instinct may be to calculate what percent 2 is of 5. But as long as you remember that “is” means “equals” and “what” means “x” you’ll be able to correctly translate the word problem into math:
So 5 is 250% of 2.
You may also be asked to figure out a percentage based on a specific occurrence. For example, if there are 200 cars at a car dealership, and 40 of those are used cars, then we can divide 40 by 200 to find the percentage of used cars at the dealership:. The general formula for this kind of calculation is:
Percent of a specific occurrence =
Converting Percents into Fractions or Decimals
Converting percents into fractions or decimals is an important GRE skill that may come into play in a variety of situations.
  • To convert from a percent to a fraction, take the percentage number and place it as a numerator over the denominator 100. If you have 88 percent of something, then you can quickly convert it into the fraction .
  • To convert from a percent to a decimal, you must take a decimal point and insert it into the percent number two spaces from the right: 79% equals .79, while 350% equals 3.5.
    • Percent Increase and Decrease
Percent Increase and Decrease
One of the most common ways the GRE tests percent is through the concept of percent increase and decrease. There are two main varieties: problems that give you one value and ask you to calculate another, and problems that give you two values and ask you to calculate the percent increase or decrease between them. Let’s have a look at both.
One Value Given
In this kind of problem, they give you a single number to start, throw some percentage increases or decreases at you, and then ask you to come up with a new number that reflects these changes. For example, if the price of a $10 shirt increases 10%, the new price is the original $10 plus 10% of the $10 original. If the price of a $10 shirt decreases 10%, the new price is the original $10 minus 10% of the $10 original.
One of the classic blunders test takers make on this type of question is to forget to carry out the necessary addition or subtraction after figuring out the percent increase or decrease. Perhaps their joy or relief at accomplishing the first part distracts them from finishing the problem. In the problem above, since 10% of $10 is $1, some might be tempted to choose $1 as the final answer, when in fact the answer to the percent increase question is $11, and the answer to the percent decrease question is $9.
Try the following example on your own. Beware of the kind of distractor we’ve just discussed.
A vintage bowling league shirt that cost $20 in 1990 cost 15% less in 1970. What was the price of the shirt in 1970?
(A) $3
(B) $17
(C) $23
(D) $35
(E) $280
First find the price decrease (remember that 15% = .15):
$20 × .15 = $3
Now, since the price of the shirt was less back in 1970, subtract $3 from the $20 1990 price to get the actual amount this classic would have set you back way back in 1970 (presumably before it achieved “vintage” status):
$20 – $3 = $17
Seventeen bucks for a bowling shirt!? We can see that . . . If you finished only the first part of the question and looked at the choices, you might have seen $3 in choice A and forgotten to finish the problem. B is the choice that gets the point.
Want a harder example? Sure you do! This one involves a double-percent maneuver, which should be handled by only the most experienced of percent mavens. Do not attempt this at home! Oh, wait . . . Do attempt this at home, or wherever you’re reading this book.

The original price of a banana in a store is $2.00. During a sale, the store reduces the price by 25% and Joe buys the banana. Joe then raises the price of the banana 10% from the price at which he bought it and sells it to Sam. How much does Sam pay for the banana?

This question asks you to determine the cumulative effect of two successive percent changes. The key to solving it is realizing that each percentage change is dependent on the last. You have to work out the effect of the first percentage change, come up with a value, and then use that value to determine the effect of the second percentage change.
We begin by finding 25% of the original price:
Now subtract that $.50 from the original price:
$2 – $.50 = $1.50
That’s Joe’s cost. Then increase $1.50 by 10%:
Sam buys the banana for $1.50 + $.15 = $1.65. A total rip-off, but still 35 cents less than the original price.
Some test takers, sensing a shortcut, are tempted to just combine the two percentage changes on double-percent problems. This is not a real shortcut. It’s more like a dark alley filled with cruel and nasty people who want you to do badly on the GRE. Here, if we reasoned that the first percentage change lowered the price 25%, and the second raised the price 10%, meaning that the total change was a reduction of 15%, then we’d get:
Subtract that $.30 from the original price:
$2 – $.30 = $1.70 = WRONG!
We promise you that if you see a double-percent problem on the GRE, it will include this sort of wrong answer as a trap.
Two Values Given
In the other kind of percent increase/decrease problem, they give you both a first value and a second value, and then ask for the percent by which the value changed from one to the other. If the value goes up, that’s a percent increase problem. If it goes down, then it’s a percent decrease problem. Luckily, we have a handy formula for both:
percent increase =
percent decrease =
To borrow some numbers from the banana example, Sam pays $1.65 for a banana that was originally priced at $2.00. The percent decrease in the banana’s price would look like this:
percent decrease =
So Sam comes out with a 17.5% discount from the original price, despite lining Joe’s pockets in the process.
A basic question of this type would simply provide the two numbers for you to plug into the percent decrease formula. A more difficult question might start with the original banana question above, first requiring you to calculate Sam’s price of $1.65 and then asking you to calculate the percent decrease from the original price on top of that. If you find yourself in the deep end of the GRE’s question pool, that’s what a complicated question might look like.
Common Fractions, Decimals, and Percents
Some fractions, decimals, and percents appear frequently on the GRE. Being able to quickly convert these into each other will save time on the exam, so it pays to memorize the following table.
Fraction Decimal Percent
0.125 12.5%
(the little line above the 6 means that the 6 repeats indefinitely, so 0.166 = .1666666666 . . .)
0.2 20%
0.25 25%
0.375 37.5%
0.4 40%
0.5 50%
0.625 62.5%