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Sometimes, as you solve a GMAT Problem Solving problem, everything seems to go smoothly. You get an answer that matches one of the choices perfectly, so you select it and move on to the next problem. But much later, when you’re reviewing the problem, you realize that you picked the wrong answer entirely. Why does this happen, and how can you stop it?
According to data from GMAT Navigator, our online platform that lets students record their answers to GMAT practice problems (click here for the premium version or here for the basic version), there are certain Problem Solving problems that most students feel very confident about. Very few students guess on these problems. However, many students get these problems wrong, despite their high level of confidence. Here’s the list of mistakes these students make most frequently.
- Off by 1: When a problem asks for a largest or smallest possible value, or when it includes inequalities, double-check your work before answering. The answer choices are often only 1 apart, and it’s easy to choose an answer that’s 1 too small or too large. For instance, what is the smallest integer that’s greater than 41/4? If you quickly divide 41 by 4 and take the integer part of the answer, 10, you’ll be off by
- Off by 100: Be careful with percentage problems, especially ones with answer choices that are similar except for the number of decimal places (0.89, 8.9, 89, 890, etc.) If you miss the word ‘percent’ somewhere in a problem, or if you forget to multiply a decimal by 100 to convert it to a percent, your answer will be off by a factor of 100. Check out our guide to approaching percent problems if you make this mistake.
- Bad unit conversion: It’s easy to make a mistake when converting between kilometers and miles, dollars and cents, or grams and kilograms. The worst culprits are problems that ask you to convert between units you may not be familiar with, or even made-up units, since your instincts won’t tell you that your answer is wrong. Always write out the entire unit conversion and check your work carefully. It’s more time-consuming, but it’ll ensure you never multiply when you should divide, or accidentally convert the wrong value. Read our guide to fast and safe unit conversions to get a handle on this issue.
- Missing the last step: You get all the way through the problem, doing all of the math correctly, and solve for the value of n. Then you choose that value and move on, never noticing that the problem actually asked you to choose the value of 2n, or n2, as your answer. To avoid this, read the entire problem, and the answer choices, before you begin writing. Before you write anything else on your paper, jot down what you’re solving for and circle it.
- Losing the units: Picture a rates & work problem that asks you to solve for the amount of time it would take two machines to complete a job when working together. Unbeknownst to you, one of the wrong answer choices is actually the rate at which the two machines are working, which is the reciprocal of the time. The answer choices don’t include units, so you have to keep track of them yourself. And be aware that the test writers might play dirty tricks with units in the text of a problem: for instance, rates are usually given in miles per hour, but I’ve seen a problem that includes a rate given in minutes per mile. Many of my students gloss right over the units, assume that the rate is written in miles per hour, and do all of the math correctly but get the wrong answer anyways. Don’t let that happen to you.
- Percent more than/less than: 120 is 20 percent more than 100, but 100 isn’t 20 percent less than 120. (Take a moment to do the math, and figure out what number actually fits there!) You can’t switch back and forth between percent more than and percent less than, even if it seems to make the problem easier; the math just doesn’t work out. Make sure that when you translate a percent word problem into math, you’re keeping the same expressions as in the original problem.
These issues were the six most common culprits that caused students to unknowingly miss GMAT Problem Solving questions. Learn them now, and double-check your work whenever you see a suspicious problem. There’s nothing wrong with missing a problem because you haven’t learned the math yet, but you never want to miss problems that you could easily have gotten right. 
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Chelsey Cooley
is a Manhattan Prep instructor based in Seattle, Washington. Chelsey always followed her heart when it came to her education. Luckily, her heart led her straight to the perfect background for GMAT and GRE teaching: she has undergraduate degrees in mathematics and history, a master’s degree in linguistics, a 790 on the GMAT, and a perfect 170/170 on the GRE. Check out Chelsey’s upcoming GMAT prep offerings here.
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