Official Guide Explanation:
Problem Solving #139




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.

Click here for an example of the PDF booklets. Click here to purchase a PDF copy.


Solution and Metadata

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).

Click here for the full list of GMAT OG12 explanations.


You should follow me on Twitter. While you're at it, take a moment to subscribe to GMAT Hacks via RSS or Email.

Total GMAT Math

The comprehensive guide to the GMAT Quant section. It's "far and away the best study material available," including over 300 realistic practice questions and more than 500 exercises!
Click to read more.