GMAT Data Sufficiency

1The five fixed answer choices

Every Data Sufficiency item offers the same five choices. Choice A: statement (1) alone is sufficient, and statement (2) alone is not. Choice B: statement (2) alone is sufficient, and statement (1) alone is not. Choice C: the two statements together are sufficient, but neither alone is. Choice D: each statement alone is sufficient. Choice E: even together the statements are not sufficient.

Memorise the wording cold. On test day you should never re-read the options — the letters alone tell you which sufficiency pattern you have just proved.

Figure. The five letters never change. Memorise the pattern each letter names so you never re-read the options on test day.

2Sufficiency, not the value

You decide only whether the statements can answer the question — not what the numeric answer is. The moment you know a unique answer is reachable (or that it is not), stop. Computing the final number wastes time the section does not give you, and it is never what the choice asks for.

That is the mindset shift that unlocks the format. A statement that "looks relevant" is not enough; a statement that pins exactly one answer is.

Figure. Data Sufficiency asks whether a unique answer is pinned — not what that number is. Stop the moment uniqueness is decided.

How to stop early

1. Name what is askedUnderline the exact question — a value, or a yes/no — so you know what "unique" means here.
2. Ask uniqueness, not the numberFor a value question: does this statement force exactly one number? If yes, it is sufficient; write the choice path, do not finish the arithmetic.
3. Leave the value blankThe answer sheet wants A–E, never the number you nearly computed.

3Evaluate each statement alone first

Test statement (1) alone, then statement (2) alone, and only combine them if neither is sufficient by itself. Never carry information from one statement into the other's individual test — that leak is exactly what turns a correct B into a wrong C, or a correct A into a wrong D.

A practical habit: cover statement (2) while judging (1), then cover (1) while judging (2). Only uncover both when both alone have failed.

Figure. Flowchart for the Data Sufficiency test order. From the question, test statement (1) alone: yes leads to A or D; no leads to testing statement (2) alone. From (2) alone: yes leads to B; no leads to combining both, which chooses C or E.

The testing order

1. Statement (1) aloneCover (2). Ask: does (1) by itself pin a unique answer? Record sufficient or not.
2. Statement (2) aloneCover (1). Forget everything (1) said. Ask the same uniqueness question of (2) alone.
3. Combine only if both failedIf either alone was sufficient, you already know A, B or D. Open both statements together only when each alone was insufficient — then choose C or E.

4Value questions need one number

When the stem asks "what is the value of…", a statement is sufficient only if it forces exactly one number. Infinitely many pairs that satisfy an equation are not enough, even when the equation is true of the unknown.

Read what is asked before you reach for algebra. A statement that gives x - y does not give x + y. A statement written as 2x + 2y = 20 is already x + y = 10 — and that alone may answer the question without its partner.

Figure. Sufficiency depends on the asked target. An open family for (x, y) can still pin a unique value of x + y.

How to test a value stem

1. Name the targetWrite the exact expression asked for — x, x + y, |x| — and refuse to compute anything else.
2. Simplify each statementRewrite (1) and (2) into the target if you can. Stop the moment the target is a single number.
3. Reject open familiesIf two different values still satisfy the statement, it is insufficient — even if both values are "reasonable".

5Yes/no questions need one consistent answer

For a yes/no Data Sufficiency stem, a statement is sufficient if it always gives the same answer — always yes, or always no. Consistency is sufficiency; positivity is not. A statement that sometimes yields yes and sometimes no is insufficient, even when "yes" is the answer you expected.

Even powers hide sign: x^2 = 9 gives x = 3 or x = -3, so "is x > 0?" is not settled. Odd powers keep sign: x^3 = 27 gives only x = 3, and the yes is locked.

Figure. Number line for statement (1) of the worked example. Tick marks at x = −3, 0 and x = 3. Under x squared equals 9 both −3 and 3 are allowed, so the yes/no question "is x greater than 0" gets no at −3 and yes at 3 — not a unique answer. Positions are to scale on a span from −4 to 4.

How to test a yes/no stem

1. Try for both answersUnder the statement alone, hunt for one case that says yes and one that says no. If both exist, the statement is insufficient.
2. Watch even powersAn even power of x (or an absolute value) usually admits two signs — check both before calling the statement sufficient.
3. Accept a consistent noIf every allowed case answers no, the statement is sufficient. "No" is a definite answer, not a failure.

6Combine only after both alone fail

Choice C is the most common wrong letter when the testing order is skipped. You open both statements, solve, get a clean number, and pick C — without noticing that one of the two statements already forced that number alone.

The rule is mechanical: never combine until each alone has been marked insufficient. If either alone works, the letter is A, B or D. Combining is reserved for the residual case where each alone leaves the answer open and the pair closes it — or fails to, which is E.

Figure. Combine only when both alone marks are insufficient (I/I). If either alone is sufficient, the letter is A, B, or D — not C.

How to avoid a wrong C

1. Mark each alone firstWrite S or I beside (1) and beside (2) before you let the two statements touch.
2. Read the pair of marksS/I → A. I/S → B. S/S → D. Only I/I opens the combine step.
3. Combine as a third testWith both open, ask uniqueness again. One value → C; still open → E.