Data Sufficiency — The Question Type Where Solving the Problem Is the Mistake

Data sufficiency is the only question type in a reasoning paper where working out the answer is a mistake.

You are given a question and two statements. You are not asked what the answer is. You are asked whether the statements are enough to find it. That is the entire chapter, and every mark lost in it is lost by forgetting that one sentence.

The options are standard and barely change between exams. Statement I alone is enough but II is not. Statement II alone is enough but I is not. Either one alone is enough. Both together are needed and neither alone will do. Even both together are not enough.

'Enough' has a strict meaning here: the statements must pin the answer down to exactly one value. Narrowing it to two possibilities is not sufficiency. That single definition decides more questions than any other idea in the chapter.

SSC CGL and CHSL, the railway exams, BPSC TRE-4 mental ability and UPSC CSAT all use this format, and it rewards discipline far more than it rewards speed.

1. Question: What is the value of x? Statement I: x² = 49. Statement II: x > 0. Which statements are sufficient?

2. Question: In how many days can Ramesh finish a piece of work alone? Statement I: Ramesh alone finishes the work in 12 days. Statement II: Suresh works twice as fast as Ramesh. Which statements are sufficient?

3. Question: Who is the tallest among A, B, C and D? Statement I: A is taller than B. Statement II: C is taller than D. Which statements are sufficient?

4. Question: On which day of the week did Rahul reach Patna? Statement I: He reached after Tuesday. Statement II: He reached on the day immediately before Friday. Which statements are sufficient?

5. Question: In which month of the year was Ravi born? Statement I: Ravi was born after March and before May. Statement II: Ravi's birthday falls in the fourth month of the year. Which statements are sufficient?

6. Question: What is the area of a rectangle? Statement I: Its perimeter is 40 cm and its length is 12 cm. Statement II: Its diagonal is 15 cm. Which statements are sufficient?

7. Question: What is the total number of students in a class? Statement I: There are 20 more girls than boys. Statement II: There are 25 boys and 45 girls. Which statements are sufficient?

8. Question: What is Meena's present age? Statement I: Meena is 6 years older than her brother. Statement II: Her brother is 18 years old. Which statements are sufficient?