Why Approximate?

I'm always encouraging my students to do more approximation and less calculation on GMAT math questions. Even those of you who practice estimating and approximating would probably benefit from doing it even more.

Here are five reasons why approximation is so important on the GMAT Quant section. Some of them are obvious, while others are barely discussed.

1. It saves time. As I always say, the GMAT is just as much a time-management test as it is a content test. If you approximate your way to easy numbers and do the math in your head, or even if you can save a step or two, you've just earned yourself more time to spend on other questions.

2. The GMAT wants you to. There's a reason you can't use a calculator on the GMAT. The test isn't about calculation, it's about thinking skills. 21 isn't that different from 20, and the square root of 2 isn't that different from 1.4, or even 1.5, depending on the context. Answer choices rarely demand precision.

3. You'll avoid mistakes. If you calculate 102 divided by 21 using long division, there's a decent chance you'll make a misstep along the way. If you find the result of 100 divided by 20, you'll get it right every time. There are almost no GMAT questions on which the lack of precision will hurt you.

4. The worst-case scenario is still pretty good. Let's say you try approximating on a hard question, and after two minutes of work, you get an answer. It's right in between two of the answer choices. It seems you've approximated too much.

But look at the benefits! If it took you two minutes to do the problem with round numbers, how long would it have taken with the original values? Maybe you would've gotten to the answer, but what about the cost in time? And even though you're stuck guessing between two choices, your chances are 50/50. That's better than the 20 percent chance you'd have if you had to guess on the last question at the end of the section.

5. It makes you think about math. Once you get out of school, "math" isn't about dreary calculations anymore. It's about relationships between numbers: fluctuations in stock prices, discounts on bulk purchases from vendors, fantasy football results.

When you approximate, you start to think about what happens when you subtly shift numbers. Sure, 101/19 is about the same as 100/20, which is about the same as 99/21. But which one is biggest? Why? You're not going to absorb those things with a calculator, but those sorts of thinking skills are what build up your aptitude for Data Sufficiency questions.

Start practicing approximation today. When you're studying, it doesn't matter whether you're right or not. Try approximating even on questions when you don't think it'll work out. See what happens. Best case, you'll discover new ways to use the technique. Worst case, you'll spend a few minutes of your life learning a bit more about numbers. Either one will have a positive long-term impact on your GMAT score.

About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.

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