Manipulating Percents

September 29, 2008

One of the most useful skills you can bring to the GRE is an ability to work with percents. For starters, you should be able to quickly translate percents to decimals and vice versa:

This comes in particularly handy on one common type of GRE Quantitative Comparison question. You may have seen something like it:

If you convert the percents to decimals and the word "of" to a multiplication sign, you're one step closer to comparing the columns:

For some people, that's all you need to do--you can quickly determine that both columns are equal to 10, so the correct answer is (C). However, there's an additional technique you can use to avoid even that much math.

Movable Decimal Points

Consider Column A: 0.2 x 50. 50 = 5 x 10, so if you wanted, you could write the whole thing as 0.2 x 5 x 10.

The order in which you multiply doesn't matter, so you can rearrange those as 0.2 x 10 x 5. Combine the first two terms, and you've got 2 x 5. That's the same as Column B!

In practice, you don't want to work through that process quite so slowly. The takeaway is that, when you're multiplying, you can move the decimal point of one of the numbers if you move the decimal point of another number in the opposite direction.

For instance:

Make up some numbers, and try it yourself! You can always check your answers with a calculator.

For more on percents, try Total GRE Math, which has a chapter devoted to percents, as well as GRE-like practice questions to show you how these concepts are tested on the exam.

Jeff Sackmann is a test-prep tutor based in New York City and the author of Total GRE Math, among other GRE and GMAT resources.

Need a better Quant score? Check out Total GRE Math.