Official Guide Explanation:
Problem Solving #137

Background

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Solution and Metadata

Question: 137
Page: 79
Difficulty: 5 (Moderate)
Category 1: Word Problems > Mixture Problems >
Category 2: Arithmetic > Descriptive Statistics > Average

Explanation: This is a weighted average problem: Manfred received 6 $750 paychecks and 20 $780 paychecks. The typical weighted average formula would look like this:

((6(750) + 20(780))/26)

But because the actual numbers don't matter in a weighted average calculation, we can subtract 750 from each of the dollar amounts, so long as we remember to add 750 to the result when we're finished:

((6(0) + 20(30))/26) = ((0 + 600)/26) = (600/26) ≈ 23

Add 750 back to the average, for an answer of 750 + 23 = 773, choice (D).

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