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Official Guide Explanation:Problem Solving #96

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: 96
Page: 165
Difficulty: 4 (Moderately Easy)
Category 1: Word Problems > Rate Problems > other
Category 2: Arithmetic > Ratio and Proportion >

Explanation: If the ratio of hydrogen to oxygen is 2:16, that means the ratio of oxygen to the total (water) is 16:(16 + 2) = 16:18. Thus, if we know an amount of water and want to find the corresponding amount of oxygen, we can set up a ratio as follows: (16/18) = ((x)/144). Cross - multiply, then look for ways to simplify without going through the drudgery of something like 144 * 16:

16(144) = 18x

16(16) = 2x (divided 9 out of both sides)

x = 16(8) = 128, choice (D).

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