GMAT Quantitative Reasoning common mistakes
GMAT Quantitative Reasoning · Understand the traps and how to avoid them.
In Problem Solving, solving to a value and then selecting that value as the answer even when the question asks for a different quantity (e.g., computing x when asked for 2x + 3, or computing a total when asked for the remainder).
Under time pressure the brain treats the first number produced as the goal, and the final read of the question stem is often skipped once a clean intermediate value appears.
What to do instead
Before computing, underline the exact quantity requested and write its symbolic form next to the work. As the last step, substitute back and compare the result to the underlined target; if they differ, do one more operation rather than selecting the first number.
Manipulating inequalities as if they were equations, especially multiplying or dividing both sides by a variable or negative expression without flipping the sign, or dropping the sign-flip when a stated variable range is negative.
Equation habits are overlearned from earlier math, and the sign-flip rule is easy to state but rarely practiced inside multi-step algebra.
What to do instead
Adopt a rule: never multiply or divide an inequality by an expression containing a variable. Instead move all terms to one side and factor, or test the boundary values and one interior value in each interval to confirm direction.
In Data Sufficiency, treating the two statements as a sequence to be solved (using statement 1's result while evaluating statement 2) instead of evaluating each statement independently against the question.
The format invites linear problem-solving, and once a value is found in statement 1 it contaminates judgment about whether statement 2 alone suffices.
What to do instead
Physically cover the other statement. Write '1 alone:' and '2 alone:' as separate lines, and only after both verdicts are recorded consider them together. Never carry a number from one line to the other.
In Data Sufficiency, answering 'sufficient' when a statement yields a single value in one scenario but permits multiple values across cases, particularly when the question involves a variable whose sign, integer status, or ordering is unconstrained.
Candidates test one convenient case (usually positive integers) and stop, so uniqueness is assumed rather than proven.
What to do instead
Run a fixed test battery on every statement: a positive value, a negative value, zero, a proper fraction between 0 and 1, and a non-integer. If any two cases give different answers to the question, mark insufficient.
Confusing 'sufficient' with 'gives the answer yes' in yes/no Data Sufficiency questions, marking a statement insufficient because it produced 'no'.
The goal feels like proving the statement true, so a definite negative reads as a failure rather than a definitive answer.
What to do instead
Reframe the criterion as 'one answer or many?' A consistent 'no' across all cases is sufficient. Write 'Y' or 'N' beside each test case; sufficiency means all letters match, not that they are all Y.
Setting up weighted-average and mixture problems with the weights reversed, or averaging two averages by simple mean instead of weighting by group size.
The arithmetic is easy once the structure is seen, so candidates rush the setup and assign the larger weight to the wrong group.
What to do instead
Before calculating, write the weight next to each group explicitly (e.g., '3 parts at 20%, 2 parts at 50%') and check that the result lies between the two group values and closer to the larger group. A result outside the interval or at the midpoint of unequal groups signals an error.
In rate/work problems, adding rates that are not in the same units, or adding times instead of rates when two workers or machines combine.
Time is the salient quantity in the problem statement, so it gets combined directly, while the correct operation is on reciprocals.
What to do instead
Convert every rate to 'jobs per hour' (or the stated unit) before combining, sum the rates, then invert at the end if time is asked. Sanity-check that the combined time is less than the fastest individual time.
In geometry, assuming a figure's appearance establishes a relationship (parallel lines, right angle, equal segments, center of a circle) that the problem text does not state.
Diagrams are drawn to look regular, and visual intuition overrides the stated givens, especially under time pressure.
What to do instead
List the given facts in words before solving and derive every relationship from that list. When a needed relationship is absent, treat it as unknown and look for the theorem or given that supplies it rather than reading it off the drawing.
Overlooking the second solution in absolute-value equations and in quadratics, reporting only the positive or larger root when the question admits both.
Taking a square root or clearing absolute value feels like a single operation, and the negative branch is mentally discarded as extraneous.
What to do instead
For every |expression| = k, immediately write two equations. For every quadratic reached by factoring, write both roots and check each against the question's constraints (sign, integer, range) before choosing.
Failing to convert units or scale factors consistently within a single problem, mixing minutes with hours, centimeters with meters, or percent with decimal in the same computation.
Unit labels are dropped once numbers are extracted, and the mismatch only surfaces as a wrong answer with no obvious cause.
What to do instead
Carry units through the arithmetic as if they were algebraic symbols and cancel them; if the final unit is not the unit requested, the setup is wrong. Convert all quantities to one system at the start, before any calculation.