Official Guide Explanation:
Problem Solving #D07

Background

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Solution and Metadata

Question: D07
Page: 21
Difficulty: 6 (Moderately Difficult)
Category 1: Arithmetic > Discrete Probability >

Explanation: The simplest way to look at this is to recognize that each member of the group has an equal chance of being chosen for any of the positions. Since there are 10 members, there is a one - in - ten chance that any member is chosen for any of the three positions.

Thus, there is a (1/10) probability that Harry is chosen to be secretary, and a (1/10) probability that he is chosen to be treasurer. Since it's impossible that he can be chosen to be both, we don't need to worry about that possibility. The probability that he is chosen for one or the other is the sum of the two probabilities:

(1/10) + (1/10) = (2/10) = (1/5), choice (E).

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