Core idea
Data sufficiency asks whether the information fixes a unique answer, rather than asking you to calculate from every statement.
Solving method
Ask whether the target answer is unique, not whether every variable is known. Test each statement alone using only the original stem, then combine if necessary.
Worked example
Find a rectangle’s area. Statement 1 gives length 5. Statement 2 gives width 3. Are both needed?
1. Length alone leaves the width unknown.
2. Width alone leaves the length unknown.
3. Together they give a unique area: 5 × 3 = 15 square units.
Read the diagram
The first two rows retain an unknown dimension, so several areas remain possible. The last row fixes both dimensions and hence a unique area of 15 square units. This is sufficiency, even before detailed calculation. A counterexample with another width disproves sufficiency of length alone.
Exam-style application
Is x positive? Statement 1: x^2=9. Values 3 and -3 give different answers, so insufficient. Statement 2: x+y=0. With unrestricted y, x can have either sign, so insufficient. Together still allow (3,-3) and (-3,3), so insufficient. Adding y<0 would force x>0, but that information was not supplied. A definite no would also count as sufficient.
Common mistakes
Test each statement alone before combining them. Carry no information from statement 1 into the test of statement 2 alone. GMAT uses its own five-choice sufficiency format; these lessons teach the logic, not a format simulation.
