Picking Numbers on the GMAT: Percents in the Answer Choices

by on October 10th, 2010

When confronted with a GMAT problem that features numerous variables or unknowns, in which you are not asked to directly solve for any of the unknowns, picking numbers for those unknowns is one of the most effective ways to solve the problem.  One particular type of problem in which you should almost always pick numbers is percent problems featuring unspecified values.  For example:

Sample problem:

In 1998, the profits of Company N were 10 percent of revenues.  In 1999, the revenues of Company N fell by 20 percent, but profits were 15 percent of revenues.  The profits in 1999 were what percent of the profits in 1998?

Unspecified value percent problems are questions in which every number in the problem is either a percent or ratio.  Remember that if even one number in the problem cannot be expressed as a percent, you will not be able to pick numbers to solve.

However, when you can express every number as a percent, then picking numbers will be the best option available.  And here is the most important piece of this technique to remember: when picking numbers on percent problems, you should almost always pick 100 as your unknown value.  Because a percent is always out of 100, by setting 100 as your initial number, you will be directly working in percents from the beginning.  Then, when you get to the end of the problem, you will already have your answer in percent terms.  And of course, it’s much easier to figure out, say, “17.2 % of 100” than it is to find “17.2 % of 68”, or another random number, so keep the math easier on yourself.  Sticking with 100 saves time and brain power overall.  (See our post on Picking the Right Numbers for ideas on what to pick for other types of problems.)

When picking numbers in percent problems, it is still important to remember the percent formulas, in order to double-check your work.  Make sure to keep in mind that percent equals final/original x 100 (or part/whole x 100) and percent change equals (difference/original) x 100.

By following this strategy you will be well on your way to answering percent problems quickly and effectively.  For more examples of this type, view our Kaplan GMAT Video on Percents in the Answer Choices.

Here’s the solution to the sample problem we included above:

In 1998, the profits of Company N were 10 percent of revenues.  In 1999, the revenues of Company N fell by 20 percent, but profits were 15 percent of revenues.  The profits in 1999 were what percent of the profits in 1998?

 

Since we have a percents question with an unspecified value in the question stem, we can pick numbers, and we should pick 100.  Pick 100 for the 1998 earnings.  That means profits were 10 (10% of 100).  Revenue in 1998 was 80 (since it fell 20%),  and profits were 15% of that revenue, or 12.

The question becomes, 12 is what percent of 10?  Or 12/10 * 100 = 120% (which is the correct answer here).

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