FAST Math for the GMAT (Part 1 of 5)

by Stacey Koprince Apr 25, 2017

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I don’t know about you, but I’m pretty lazy when it comes to doing math on paper. Blame constant access to Excel and the calculator on my phone…but I’m completely over doing math on paper.

If you give me a problem that’s going to require half a page of calculations…well, I’m not going to want to do that problem. But on the GMAT Quant section, I don’t get a calculator, so how can I still get a 99th percentile score while staying true to my lazy-math desires?

Let’s do some Fast Math!

Principle #1: Don’t Do Math till You HAVE to

Sure, write down that equation in the question stem. And, sure, set up the math that you would need to do. BUT. If any of the math looks annoying, don’t do it yet. Be patient. Wait a little longer to see whether you really need to do it.

For instance, I want you to tell me what 5/12 of 81 is. Oh, and then I want you to multiply that result by 240.

5/12 of 81…hmm. 81 isn’t divisible by 12…but the two numbers do share 3 as a factor, so I can at least simplify a bit and then maybe I’ll need to do some longhand multiplication and division…

Stop right there. On the GMAT, if I’m doing longhand multiplication or division, I’ve missed something. Back up. Look at the whole problem.

Here’s the full math that I asked you to do:

⁵₁₂  x  ⁸¹₁  x  ²⁴⁰₁

That 81 can’t completely cancel out the 12…but the 240 can! Check it out:

⁵₁₂  x  ⁸¹₁  x  ²⁴⁰₁  =  ⁵₁  x  ⁸¹₁  x  ²⁰₁

Now, let’s see, 5 times 81…

Wait! When multiplying a string of numbers, always look to pair 5’s and 2’s first. Why? 5’s and 2’s create 10’s (or multiples of) and those are a whole lot easier to multiply into the rest of the numbers.

(5)(20) = 100

(100)(81) = 8,100

Done!

Whenever you’ve got a multi-step math problem, try to set up as much as you reasonably can and look to simplify before you even think about solving.

Next, when you can’t simplify any further, don’t just do the remaining math left to right. Take a moment to look at the big picture—see whether you can rearrange or approach the math in a way that makes the calculations easier.

There you have it: Principle #1 for Fast Math. Next time, we’ll discuss Principle #2—see you then! 📝

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Stacey 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.