Official Guide Explanation:
Problem Solving #101




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Solution and Metadata

Question: 101
Page: 166
Difficulty: 5 (Moderate)
Category 1: Word Problems > Work Problems > 2 inputs

Explanation: Whenever you're given two different rates for the same job (in this case, filling a pool) and you're looking for the combined rate at which the job could be done, use the following equation: ((ab)/(a + b)), where a and b are the times required for each of the two separate entities to do the job. In this case, a = 9 and b = 5. Applying the equation: ((9(5))/(9 + 5)) = (45/14). 45 is a little more than three times 14(3 * 14 = 42), so the correct answer is 3.21, choice (D).

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