Official Guide Explanation:
Problem Solving #70

Background

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Solution and Metadata

Question: 70
Page: 70
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Properties of Integers > Evens and Odds
Category 2: Arithmetic > Descriptive Statistics > Average

Explanation: Given that the least integer in S is 7 more than the least integer in T, we can pick some values to get a better grasp of what's going on. If we say the least in S is 1, then S is {1, 3, 5, 7, 9, 11,13, 15, 17, 19}. That means the least in T is -6, so T is {-6, -4, -2, 0, 2}. The average of either one is the midpoint of the least and greatest integers, so in S, it's 10; in T, it's -2. The difference is 12, choice (D).

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