GMAT Tip: Doing 'Mental Math'

Use estimates where possible and simplify complex calculations. Photograph by Amy Eckert

The GMAT Tip of the Week is a weekly column that includes advice on taking the Graduate Management Admission Test, which is required for admission to most business schools. Every week an instructor from a top test-prep company will share suggestions for improving your GMAT score. This week’s tip comes from Mike McGarry, lead GMAT content creator at Magoosh.

While the new Integrated Reasoning section on the GMAT includes an on-screen calculator, the Quantitative section remains calculator-free. Math without a calculator strikes at least a modicum of fear into the hearts of all but the nerdiest of math geeks (including, yes, yours truly). Here are three tips to make the calculator-free Quantitative section a less-bitter pill to swallow.

1. Practice mental math every day between now and the GMAT. Do math in everyday life. Force yourself to calculate the tip, sum your grocery bill while on line, and so forth. Work that “mental math” muscle as hard as you can now, so using it is not such a shock on test day.

2. Estimate. On GMAT problems, answer choices that are widely spaced are a clarion call to estimate. If you work 42 hours at a rate of $19.74/hr, how much to you make? Well, if you approximate hours as 40 and rate as $20 per hour, your earnings have to be close to 40 x 20, or $800. If the answer choices are widely spaced, then pick the answer closest to $800 and you are golden. Again, practice this as much as possible before the GMAT, so that it becomes a comfortable move on test day.

3. Cancel before you multiply. Sometimes, you can’t estimate: you have to calculate an exact answer. Suppose you sell 54 shares of stock ABC, at $32 per share, and then you use that money to buy stock XYZ, currently selling at $48 a share. How many shares of XYZ can you buy? Well, clearly, the money you make is 54 x 32, and then you will divide that by 48, so (54 x 32)/48. The abysmal mistake that so many people make is: step #1 = calculate the value of 54 x 32. That is a disaster. That gives you a four-digit number, which you then will have to divide by 48 without a calculator. Nightmare.

Instead, ask yourself: Before multiplying, are there any common factors to cancel? Both 54 and 48 are divisible by 6, so (54 x 32)/48 easily becomes (9 x 32)/8.  Now, notice that 32/8 = 4, so (54 x 32)/48 = (9 x 32)/8 = 9 x 4 = 36.  You can buy 36 shares of XYZ, and you never had to do more than two-digit arithmetic.

No calculator = no problem.

Mike McGarry scored in the 99th percentile on the GMAT. He is an expert in standardized test preparation, and has been a teacher for over 20 years. McGarry earned both a bachelor’s degree in physics and a master’s in comparative religion from Harvard University.

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