


Writing Equations
For some questions on the Math IC test, you’ll need to
translate the problem from a language you’re used to—English—into
a more useful, albeit less familiar language. We’re talking about
the language of math, of course, and one of your major testtaking
responsibilities is being able to write an equation based on the
pertinent information you’re given by a problem.
In other cases, you’ll simply be asked to find an expression
for a certain quantity described in a word problem. The best way
to learn how to do these things quickly and effectively is to practice.
Here’s a sample problem:

To start with, you can write r + b =
50, where r is the number of red marbles, and b is
the number of blue marbles in the sack. This equation tell us that
all of the 50 marbles in the sack are either red or blue.
Now that we have a starting equation, you need to decipher
what exactly the question is asking for. This problem gives a clearcut
request: how many blue marbles are in the sack? You must therefore
find the value of b.
Unfortunately, you can’t do that with just this equation.
More information needs to be incorporated. For example, use the
knowledge that there are 20 more red marbles than blue marbles.
This part of the word problem can be written in the form of an equation
as r = b + 20. You could also write b = r –
20 to signify the same concept.
Let’s list the two equations we have so far:
Using both of these equations, you can solve for b. After
a little manipulation, which we’ll cover in the coming sections,
you’ll find that b = 15 (and r =
35). Don’t worry about the solution for now—just focus on how we
translated the word problem into equations that lead to the solution.
That problem was easy. Here’s a harder one:

According to the problem, we need to find an expression
(notice, not an equation) for the price in dollars of 35 oranges.
The key to a problem like this one is working step by step. First,
find out how many of the 35 oranges aren’t free of charge:
because f is the number
of oranges that are free, and 35 > f.
Next, find the price of those oranges:
But wait. Did you notice that the question asked for the
price of 35 oranges in dollars? The writers of
the Math IC are a clever bunch, if not a little sneaky. They figure
that a good number of testtakers will see only the word price,
and they will not notice what units are asked for. Be careful not
to fall into their carefully laid trap.
We know there are 100 cents per dollar, so we can easily
convert the price by dividing by 100:
Before we move to another problem, note that the variable r didn’t
appear anywhere in the answer. Egad! It is yet another attempt (and
a common one at that) by those devious testwriters to lower your
score. You may come across many problems, especially word problems,
in which extraneous information is provided only to confuse you.
Just because a variable or number appears in a problem doesn’t mean
that it will be useful in finding the answer.
Here’s one last problem:

This word problem is long and complicated, but you need
to carry out just four steps to solve it:

Gus must buy
^{x}/ _{y} cans of paint to cover his house.  This
will cost him
^{xp}/ _{y} dollars.  The jeans Gus buys cost 10d dollars.
 Thus,
the difference, in dollars, between the cost of the paint and the
cost of the jeans is
^{xp}/ _{y} – 10d.
For the rest of this chapter, we’ll constantly be converting
word problems into equations. If you’re still uncomfortable doing
this, don’t worry. You’ll get a lot more practice in the sections
to come.
