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## Official Guide Explanation:Problem Solving #84

Background

This is just one of hundreds of free explanations I've created to the quantitative questions in The Official Guide for GMAT Quantitative Review (2nd 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: 84
Page: 72
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Descriptive Statistics > Average
Category 2: Algebra > Linear Equations-One Unk >
Category 3: Arithmetic > Decimals >

Explanation: It may not read like one, but essentially this is a weighted average problem. There are six numbers, the group of which average to 8.5. Five of them (the six, minus the discarded number) average to 7.2. The other one "averages" a value we don't know--it's the value of that final number that we're looking for. We can set up the equation like this:

((5(7.2) + 1(x))/6) = 8.5

36 + x = 8.5(6) = 51

x = 51 - 36 = 15, choice (E).

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