Official Guide Explanation:
Data Sufficiency #134




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Solution and Metadata

Question: 134
Page: 284
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Descriptive Statistics > Standard Deviation
Category 2: Arithmetic > Descriptive Statistics > Average

Explanation: To find standard deviation, we need to know one of two things: every number in a set, or each number's distance from the mean.

Statement (1) is insufficient. Knowing the mean of a set isn't enough--the idea of standard deviation is to measure how closely packed or widely dispersed the numbers are around the mean. We have no information about that here.

Statement (2) is sufficient. We don't know the exact numbers, but since all of the numbers are the same, we know each number's distance from the mean: 0. The standard deviation of the set, then, is zero. Choice (B) is correct.

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