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## Official Guide Explanation:Data Sufficiency #134

Background

This is just one of hundreds of free explanations I've created to the quantitative questions in The Official Guide for GMAT Review (12th ed.). Click the links on the question number, difficulty level, and categories to find explanations for other problems.

These are the same explanations that are featured in my "Guides to the Official Guide" PDF booklets. However, because of the limitations of HTML and cross-browser compatibility, some mathematical concepts, such as fractions and roots, do not display as clearly online.

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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