2019 AMC 10 B

Complete problem set with solutions and individual problem pages

Problem 2 Easy

Consider the statement, "If n is not prime, then n -2 is prime." Which of the following values of n is a counterexample to this statement? (2019 AMC 10B Problem, Question#2)

  • A.

    11

  • B.

    15

  • C.

    19

  • D.

    21

  • E.

    27

Answer:E

Since a counterexample must be value of n which is not prime, n must be composite, so we eliminate A and C. Now we subtract 2 from the remaining answer choices, and we see that the only time n-2 is not prime is when n=\left( \text {E}\right)27.

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