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2019 AMC 10B Problem 2

Problem 2 of 25EasierNumber TheoryLogic

Consider the statement, “If nn is not prime, then n−2n-2 is prime.” Which of the following values of nn is a counterexample to this statement?

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Solution

We need nn to not be prime, so nn can only be 15,21,27.15,21,27. Then, n−2n-2 must be not prime, leaving just 27.27. Thus, the answer is E .
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Tagged: counterexample · prime

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