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## Official Guide Explanation:Problem Solving #139

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: 139
Page: 172
Difficulty: 3 (Very Easy)
Category 1: Arithmetic > Decimals >
Category 2: Arithmetic > Percents > other

Explanation: It's easy to find 10% of a number; just a little harder to determine 15%. 10% of \$35.50 is \$3.55, so in that case the total amount paid is \$35.50 + \$3.55 = \$39.05, which means we can eliminate choices (A), (D), and (E). (It also looks very likely that the correct choice is (B), since the lower bound of that choice is so close to what we just figured out.) To find 15%, add half of \$3.55 to \$3.55, or \$1.75 (let's forget about the extra two and a half cents--the choices are round numbers) to \$3.55, for a total tip of \$5.30, for a total amount paid of \$40.80. If the range is between about \$39.05 and \$40.80, the correct answer must be, as we suspected, (B).

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