FAST Math for the GMAT (Part 4 of 5)


Manhattan Prep GMAT Blog - FAST Math for the GMAT (Part 4 of 5) by Stacey Koprince

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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 approximate, always do so! But even when they don’t, you may be able to estimate.

Train yourself to glance at those Problem Solving answer choices periodically while you work 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.

Open up your Official Guide right now and flip to the Problem Solving chapter (chapter 5 in the big book). 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.

Some of my favorites from the 2016 edition of the Official Guide (OG2016):

#105 (page 167)

#116 (page 169) (This one tells you that you can estimate)

#169 (page 176) (Can knock out two answers with some general estimation)

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

Now, these guys 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.

Take a look at #135 (page 172) in OG2016. Can you tell whether it should be an increase or decrease? What about #136 on the same page: can you figure out whether it should be more or less than half?

Start looking for opportunities to estimate—certainly when the problem asks for an approximate answer, but sometimes even when it doesn’t.

How else would you prefer to do different kinds of FDP calculations? Share your ideas in the comments. And join us next time for the 5th installment of our Fast Math series. 📝

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stacey-koprinceStacey Koprince is a Manhattan Prep instructor based in Montreal, Canada and Los Angeles, California. 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.

  1. ptl_u February 15, 2018 at 2:52 am

    Hi Stacey,
    You mentioned above “#116 (page 169) (This one tells you that you can estimate) from OG 2016”

    I don’t see how can you use the approximation method to solve that one.

    Would you please explain how is it approximation problem?