Official Guide Explanation:
Problem Solving #96

Background

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Solution and Metadata

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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