How to Review a Data Sufficiency Question
You’ve heard it here before: reviewing practice problems is even more valuable than doing them. Here’s a step-by-step guide to reviewing your GMAT Data Sufficiency problems.
Which Data Sufficiency questions should I review?
If you had infinite time, you’d want to review every Data Sufficiency problem you do. But, let’s be realistic! You have my permission to not review these types of problems:
- Easy problems that you got right with no stress
- Problems with content that you haven’t started studying
- The very hardest problems (well above your goal score level)
One note on that first bullet point: reviewing easy problems is still worthwhile, and if you have time, you should dive into the easy problems. But, if you’re pressed for time, focus on reviewing these problems:
- Problems that are moderate to “moderately hard” in difficulty
- Problems you got right, but spent too much time solving or didn’t feel confident about
- Problems you got wrong despite having already studied the material
- Any problem that surprised you or taught you a lesson about the GMAT
Don’t ever spend a whole study session reviewing one or two super-tough problems. I know it’s infuriating to walk away from a problem without understanding it. But, walking away from a low-value problem is a victory, not a defeat. Spend your review time on problems that will teach you practical lessons you can use right now. If you need the material from that 800-level problem in the future, you can always go back and review it then.
Okay, so you have a list of problems to review. What does reviewing a problem actually look like?
Reviewing a Data Sufficiency problem
The first step is to redo the problem. However, you’re going to do the problem differently this time. When you redo while reviewing, don’t use a timer. Feel free to use any outside resources you’d like, such as a calculator or the All the Quant Guide. That’s right — you’re allowed to “cheat” as much as you want! The only thing you can’t use is the explanation. I’ll say that again, because it really matters:
DO NOT READ THE EXPLANATION (YET)!
When you read the explanation too soon, you’re robbing yourself of a chance to solve the problem. And you’ll learn more from solving the problem—even if it’s a struggle and even if you need to cheat!—than you will from watching someone else solve it.
If you get stuck, check the explanation, but stop reading as soon as it gives you a good clue. Then, go back to your own paper and try to finish the work yourself. If you absolutely can’t get anywhere, even with multiple clues from the explanation, the problem might be too hard right now. Save it for later — and if this happens repeatedly with the same type of problem, that’s a sign that you should review that topic!
Once you’re satisfied with your own redo of the problem, feel free to read the rest of the explanation. Take note of anything that the explanation did that you didn’t do, or vice versa. Sometimes, the explanation can point you in the direction of a faster or simpler solution.
Learning from a Data Sufficiency problem
Now, it’s time to take some notes. But you aren’t just going to write down how to solve the problem. That would help you remember how to solve this exact problem, but that’s not the point. Here’s what your goals really are at this stage:
- Learn (at least) one thing about the GMAT.
- Learn (at least) one thing about yourself.
Before we keep going, let’s do an actual Data Sufficiency problem, so we’re all on the same page. Here’s one from mba.com’s GMATPrep:
A bookstore that sells used books sells each of its paperback books for a certain price and each of its hardcover books for a certain price. If Joe, Maria, and Paul bought books in this store, how much did Maria pay for 1 paperback book and 1 hardcover book?
(1) Joe bought 2 paperback books and 3 hardcover books for $12.50.
(2) Paul bought 4 paperback books and 6 hardcover books for $25.00.
Do the problem now! Next, we’ll walk through which notes you should take in your problem log.
A Data Sufficiency problem has two different things you need to analyze while solving: the question stem, and the statements. Take some notes on each.
First, write down anything that stands out about the question stem, and what you think you should do when you see those things in a Data Sufficiency question. Your exact notes will vary depending on your strengths and weaknesses, how you solved the problem, and why you’re reviewing it. Here are some examples.
Student 1 spent too long on this problem because she dove into the statements without understanding the problem. She didn’t realize, until too late in the process, that solving with equations would have been much simpler. Here are her notes on the question stem:
Word problem = turn into variables BEFORE dealing w/ statements!
Student 2 read the question stem and assumed she’d need to solve for the price of each book individually, instead of solving for the sum of two prices. Here are her notes:
Look for “combos” where the ? asks for a sum: hardcover AND paperback price = H + P
Could still solve even without H or P separately
Write down the exact question, don’t assume!
Student 3 got this one right, but still thought it was an interesting problem! Here are her notes:
Q is “what is H + P” : look out for tricky statements that tell you the sum, like “2H + 2P = 10”!
Next, go through the statements one at a time. Here’s where things really get interesting. If you missed the problem, then you either thought an insufficient statement was actually sufficient, or thought a sufficient statement was actually insufficient. Figure out where things went wrong first: which statement(s) did you analyze incorrectly? (For a very deep dive into these two types of mistakes, check out our articles on type 1 errors and type 2 errors.)
If you thought a statement was sufficient and you were wrong, figure out why you thought it was sufficient. Then, in your notes, prove that it was insufficient. That will probably involve writing down specific cases, even if you wouldn’t take the time to do that on test day. When you’re reviewing, take all the time you need!
If you thought a statement was insufficient and you were wrong, figure out why you thought it was insufficient. Then, prove that it was sufficient. That means demonstrating to yourself, in your notes, exactly how the statement helps you answer the question, and what answer it leads to.
Student 1 got the problem right eventually, but realized when reviewing that she didn’t need to test cases. Here are her notes on the statements:
DS word problem: turn into math equations and see if the algebra is easy BEFORE you think about cases…cases work but take a long time!
Student 2 made the first type of mistake when she put the statements together. She thought the statements were sufficient together, but they actually weren’t. Let’s see her notes:
Statements were insufficient together! I thought they were sufficient b/c there were two equations and two variables in the question.
Proof: statements simplify to the same thing, 2p + 3h = 12.5. If the statements are the same, C CAN’T be the right answer. The answer must be either D or E. Word problem to algebra = check to make sure the statements aren’t identical, don’t assume C if it seems easy!
Student 3 got this one right, but she’s still taking notes so she remembers any tricks this problem tried to play! Here are her notes.
DS linear equations: simplify statements to double check that the statements aren’t = to each other. If 2 statements are different, you can solve for two variables. But if they’re the same, you can’t solve at all (unless the statement is the same as the question.)
By the way, the answer to the question was (E). I didn’t tell you until now because of exactly what I wrote earlier — the more you play around with a problem on your own before letting somebody else tell you what to do, the more you’ll remember about it later.
What do I do with all of these notes?
From each problem you’ve reviewed, you’ve learned something about the GMAT—and you’ve hopefully learned something about how you tend to approach problems and the mistakes you make most often. What do you do with all of this information?
GMAT instructor Elaine Loh suggests a method in this article that everybody should try out. For each problem you learned something from, set a date about two weeks in the future to redo it again. For instance, you could have a study session once per week where you redo all of the old problems from a couple of weeks ago. Write down these dates in your problem log, and build a habit of checking it regularly and actually redoing the problems you’ve listed. Also, she recommends that when you have a few minutes of spare time, you simply read over the notes you’ve taken in your problem log. No need to do anything fancy — just quickly reread your takeaways once or twice a week, to jog your memory of the interesting DS problems you’ve reviewed so far.
If Data Sufficiency is a strong area for you, it’s okay to keep your problem logging low-key. Just jot down a couple of notes on problems you missed (unless they’re way too hard!). But if you want to improve your DS performance, be more aggressive in reviewing DS problems for a couple of weeks, and you may notice a change in how you think about these problems.
Want some more GMAT review tips? Check out these posts.
- How to Review a GMAT Sentence Correction Question
- How to Review a GMAT Critical Reasoning Problem
- How to Review a GMAT Reading Comprehension Question
Don’t forget that you can attend the first session of any of our online or in-person GMAT courses absolutely free. We’re not kidding! Check out our upcoming courses here.
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 170Q/170V on the GRE. Check out Chelsey’s upcoming GMAT prep offerings here.