This page is part of my unofficial solutions manual to the GRE Paper Practice Book (2e), a free resource available on the ETS website. They publish the questions; I explain the answers. If you haven’t worked through the Practice Book, give Section 6 a shot before reading this!

6.10: Dueling Copiers

This is a special kind of algebra problem called a work-rate problem. There is a basic procedure for these that we can follow every time. It boils down to two steps:

• Figure out the work rate of each machine (worker, factory, etc.).
• Apply those rates to solve for the target quantity.

In this problem, we have enough information to calculate each machine’s work rate directly:

What we really want to know, however, is the difference in the output of the two machines if each one runs for a specified amount of time:

So we plug in the times (which the problem provides) along with the rates we calculated:

The difference in output is

so the correct answer is (B).

Math Review Reference

For more on this topic, see the following section of the GRE Math Review:

• 2.7: Applications (pp. 25-27, see esp. Example 2.7.4)