FAST Math for the GMAT (Part 4 of 5)
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We’re up to part 4 of our series on Fast Math for the GMAT. If you’re seeing this for the first time, start with part 1 and work your way back here.
Let’s dive right in.
Principle #4: Estimate…and not just when they tell you to
And that brings us to awesome estimation. Some problems ask you straight up, “Approximately how far has the train gone when…” When they tell you that you can estimate, always do so! But even when they don’t, you may be able to estimate.
While solving, train yourself to glance at those Problem Solving answer choices periodically to see how far you really need to go. The correct answer isn’t the actual number…the correct answer is just A, B, C, D, or E. Who cares how you get there?
In general, if you have numerical answer choices that are decently far apart, you can often estimate at some point in the problem⎯possibly right from the beginning or possibly a little farther in, depending upon the nature of the problem and how far apart the answers are.
Also, how rough can your estimation be? Again, glance at those answers. The farther apart they are, the more loose you can be. You’ll need to practice this, like any skill, so that you know how far is too far. As you gain experience, you’ll start to understand both when and how much you can confidently estimate your way to the answer.
Have you bought the Official Guide yet (from the official makers of the GMAT)? If so, open it up to the Problem Solving chapter and start scanning down the answer choices until you find some that look decently far apart, or look for the word approximately in the question. Then see whether (and how) you can estimate.
As you get better, add some variations into the mix. For instance, one problem might have these five answers:
(A) −2
(B) −1
(C) 0
(D) 1
(E) 2
These answers don’t look all that far apart…but you may still be able to estimate! Two are negative, two are positive, and one is 0. If you can estimate enough to tell that the answer must be negative, then you have a 50/50 shot at getting this right, even if you don’t have enough time or don’t know how to do the problem for real.
Or maybe you’ll see this mix of answers on a problem involving proportions, probabilities, or similar:
(A) 1⁄5
(B) 1⁄3
(C) 1⁄2
(D) 3⁄5
(E) 2⁄3
Can you estimate enough to tell whether the proportion or probability should be more or less than half? Let’s say that you can tell that it’s less than half. The choice is then between (A) and (B). Choice (A) is only 20%, while choice (B) is about 33.3%. Next, see whether the details of the problem allow you to estimate whether the value will be less than or more than 25%. If you can tell that, you’ll know whether the answer is (A) or (B). (And, if not, you’ve still narrowed it down to two answers.)
Start looking everywhere for opportunities to estimate⎯certainly estimate when the problem asks for an approximate answer, but estimate wherever it’s reasonable to do so, even if the problem doesn’t explicitly tell you that you can estimate.
Join us next time for the 5th installment of our Fast Math series.
KEEP READING: FAST Math for the GMAT (Part 5)
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Stacey Koprince is a Manhattan Prep instructor based in Montreal, Canada. Stacey has been teaching the GMAT, GRE, and LSAT for more than 15 years and is one of the most well-known instructors in the industry. Stacey loves to teach and is absolutely fascinated by standardized tests. Check out Stacey’s upcoming GMAT courses here.