Data Sufficiency: Stop SOLVING and Start DECIDING

By EMPOWERgmat on March 28, 2017

Here is a scene I watch play out every single week. A Test Taker hits a Data Sufficiency question, grabs the pencil, sets up two equations, solves for x, checks the work, writes “x = 40” in a neat little box and THEN looks at the answer choices. Two and a half minutes gone. The question never asked what x was.

That habit is costing you points. Not because your math is wrong. Because your math is UNNECESSARY.

What Data Sufficiency actually asks

A quick refresher, because the format is the whole point. The GMAT Quant section gives you 37 questions in 75 minutes. Those questions are a mix of Problem Solving (regular math questions with five possible answers) and Data Sufficiency, or DS. A DS question gives you a question and two facts, called Statement 1 and Statement 2. Your job is NOT to answer the question. Your job is to decide whether those facts give you enough information to answer it. “Sufficient” just means “enough to get exactly ONE answer.”

The five answer choices never change:

(A) Statement 1 alone is enough, but Statement 2 alone is not.
(B) Statement 2 alone is enough, but Statement 1 alone is not.
(C) Both statements together are enough, but neither one alone is.
(D) Each statement alone is enough.
(E) Even both statements together are not enough.

Read that list again. Not one of those choices is a number. There is no choice that says “x = 40.” So when you grind the arithmetic all the way to the end, who exactly are you doing it for?

Why smart people keep solving anyway

Stop reading for a second and think about WHY you finish the math. Don't read another word until you have a reason.

Here is what I usually hear. School trained you to finish. Every math test you ever took rewarded the final number. Finishing feels safe. Stopping halfway feels like cheating, or like you might be wrong. So you keep going “just to be sure.”

That feeling is your pride and your habits talking, not your strategy. The GMAT is a critical thinking test that uses math as the measuring stick. In DS, the skill being measured is the DECISION, not the calculation.

And it adds up. Say you face a dozen DS questions on Test Day and you spend an extra 40 seconds finishing math on each one. That is 12 × 40 = 480 seconds. That is 8 minutes. Would an extra 8 minutes make the end of your Quant section a little calmer? I'll bet it would.

A runner crosses a finish line tape and keeps sprinting far past it, while a second runner stands calmly just past the tape catching their breath. — Once you know you COULD get the answer, you are done. Everything after that is extra distance.

Watch the difference

Try it

A store sold 120 shirts. Some sold for $15 each and the rest sold for $25 each. How many shirts sold for $25?

(1) The store took in a total of $2,200 from the shirts.

(2) The store sold 40 more of the $15 shirts than the $25 shirts.

Statement 1 alone is enough, but Statement 2 alone is not.
Statement 2 alone is enough, but Statement 1 alone is not.
Both statements together are enough, but neither one alone is.
Each statement alone is enough.
Even both statements together are not enough.

Show the answer

Notes first. Call the $15 shirts F and the $25 shirts T. The stem gives you F + T = 120. The question is: what is T?

Statement 1. It gives 15F + 25T = 2,200. Now you have two different equations with two unknowns, and one is not just the other multiplied by a number. That gives exactly one value of T. STOP. Sufficient. Write it down and cross off B, C and E. You are down to A or D.

Statement 2. It gives F = T + 40. Again, two different equations, two unknowns, one value of T. STOP. Sufficient.

Each statement alone works, so the answer is D.

What you did NOT need to do: solve. For the record, T = 40 either way (80 shirts at $15 is $1,200, plus 40 shirts at $25 is $1,000, for $2,200 total). But that number earned you zero extra points.

The trap: most people solve Statement 1 all the way, then solve Statement 2 all the way, burning two or three minutes. Others feel nervous about trusting one statement and pick C because more information feels safer.

The question is part of the data

Deciding is not just about stopping early. It is about knowing EXACTLY what you are being asked, so you know what “enough” looks like.

Try it

What is the value of a + b?

(1) 3a + 3b = 21

(2) a − b = 1

Statement 1 alone is enough, but Statement 2 alone is not.
Statement 2 alone is enough, but Statement 1 alone is not.
Both statements together are enough, but neither one alone is.
Each statement alone is enough.
Even both statements together are not enough.

Show the answer

Notes first. Write the question down: a + b = ? You do NOT need a or b on their own. You need the sum.

Statement 1. Divide everything by 3 and you get a + b = 7. That is the exact thing the question asks for. Sufficient. You are down to A or D.

Statement 2. TEST VALUES. If a = 1 and b = 0, then a − b = 1 and a + b = 1. If a = 2 and b = 1, then a − b = 1 and a + b = 3. Two different sums, so NOT sufficient.

The answer is A.

The trap: C. People see two unknowns, decide they need two equations, combine the statements to get a = 4 and b = 3, and feel great about it. They did correct math and still got the question wrong, because they answered a question nobody asked.

The mistakes I see, and what to do INSTEAD

Solving all the way to the end. You finish math that earns nothing. INSTEAD: ask “could I get exactly one answer from here?” The moment the answer is YES (or a clear NO), stop and decide.
Reading the statements before you understand the question. Then you don't know what “enough” means. INSTEAD: write the question in your notes first. Simplify it if you can. “Is 2x > 10?” is really “Is x > 5?”
Looking at both statements at once. Statement 2 leaks into your thinking about Statement 1. INSTEAD: handle one at a time. Write “AD” on one line and “BCE” under it. If Statement 1 works, cross off BCE. If it doesn't, cross off AD. Then deal with Statement 2 as if Statement 1 never existed.
Picking C because more facts feel safer. C is not a comfort blanket. INSTEAD: give each statement an honest, separate test before you ever put them together.
Testing one value on a yes/no question and calling it done. One value proves nothing. INSTEAD: TEST VALUES on purpose, trying to get a YES and a NO. If you can get both, it is not sufficient. And remember: “always NO” is just as sufficient as “always YES.”
Doing it all in your head. That is where the silly mistakes live. INSTEAD: take notes on every question. The stem, the question, each statement, your AD/BCE grid. Every time.

This is a training problem, not a knowledge problem

You probably already know the math in most DS questions. Reading another chapter about DS will not change your habits, which is why a book heavy study plan tends to stall right here. The fix is reps, done the right way, on questions you have already seen.

And don't just count right and wrong. Look at WHAT you did to get there. If you got a DS question right in three minutes when it needed one, that is still a problem, and you need to actually analyze your work to see it.

So you have to decide what you want MORE: the comfort of seeing the final number, or a higher Quant score.

Your challenge for today

Pull out 10 DS questions you have already done. You can use the official practice materials or anything with real DS questions. Redo every one with a pencil and your AD/BCE grid. On each question, draw a line in your notes at the exact moment you could have stopped. Then look at how much work sits BELOW that line.

I'll bet it is more than you think. Now go train yourself to stop there.