
Fractions
Fractions are ubiquitous on
the SAT Math section (there’s an SAT word for you to
chew on). A fraction shows the relationship between a part of something and
the whole of something. Here’s what a fraction looks like: .
A fraction is composed of two numbers, a numerator, and
a denominator. The numerator (the part)
is above the fraction bar, and the denominator (the whole)
is below it. So in our example, 2 is the numerator, and 3 is the denominator.
What if we multiply both the numerator and the denominator
of by 4? We would end up with:
Because we multiplied both parts by the same numbers,
the two fractions ( and )
have the same parttowhole relationship. These fractions are equivalent fractions,
which means they equal each other.
Suppose we decide that is
too bulky. We know that 8 and 12 have an LCM of 4—remember that
term?—so we can whittle that down to a slimmer
fraction:
Taking a fat fraction and dividing out similar terms from
both the numerator and denominator is called reducing a
fraction.
Reducing fractions and making equivalent fractions are
necessary skills, because you often have to perform these tasks
before adding, subtracting, multiplying, or dividing fractions.
Don’t simply take our word for it. Review the following sections.
Adding and Subtracting Fractions
We’ll start simple. Fractions with the same denominators
are the easiest. You need only add or subtract the numerators:
No rocket science there. Fractions with different denominators
are one degree trickier. Before adding or subtracting them, you
first need to make sure that the denominators are the same.
For instance, if you need to perform the operation ,
first you have to equalize the denominators of these two fractions.
You do this by finding the lowest common multiple of denominators
3 and 15. This number is the lowest common denominator,
or LCD.
Because 15 is a multiple of 3, the LCD is 15. So all you
need to do is multiply 3 and 5 to get 15. However, if you increase
the denominator by 5, you need to increase the numerator by 5 too.
Remember: all fractions are the parts of a whole:
Now you can get your subtraction on:
Suppose the LCD is difficult to find, such as when you
have 12 and 18 in the denominator. You could simply multiply 12
and 18 and use the product as a common denominator. This gives you
some big fractions that you have to reduce. Working with big numbers
increases the possibility of errors, so it’s best to find the LCD
before proceeding.
Multiplying and Dividing Fractions
Compared to adding and subtracting with different denominators,
multiplying fractions is easy. You just find the product of the
fractions’ denominators and numerators:
Sometimes you can crosscancel numbers and
make multiplying even simpler. In the example below, the 5 in the
numerator takes out the 5 in the denominator.
Division of fractions is just like multiplication of fractions
with a little twist. When dividing, you flip the second fraction,
then multiply:
One key concept to remember is: when you multiply a fraction
by itself, the resulting fraction is less than the original fraction.
For example:
The fraction is smaller than .
WeirdLooking Fractions
Not every fraction on the SAT is in the buttondown conservative
style. Many items are designed to freak you out. To do this, the
SAT uses mixed numbers and complex fractions.
We’ll cover complex fractions first.
For complex fractions, you have to calculate a fraction
of a fraction:
See that bar between the 3 and the 1? It’s a
fraction bar, but you can also think of it as a division sign. That
bar holds the key to solving complex fractions. Take the fraction
above the bar and divide it by the fraction below the bar. You just
learned how to divide fractions, so this shouldn’t be tough:
Simple enough. On to mixed numbers.
A mixed number, or improper fraction,
includes both an integer and a fraction. For example, is
a mixed number. Mixed numbers are impenetrable to simple addition
and subtraction, so you need to do one of two things before you
work with them:
 Convert them into proper fractions.
 Use your calculator and change them to decimals.
Converting improper fractions is pretty
simple. Start with a mixed number, such as .
Multiply the integer 4 by the denominator 7, then add that product
to the numerator 2 and place the new number over the old denominator:
Remember that on the gridin section, you cannot
write a mixed number as your answer because the computer reads as .
That’s why converting mixed numbers becomes so important. Of course,
you can also just convert the whole mess into a decimal.
