2019 AMC 10B Problem 2 — Solution | LIVE by Po-Shen Loh

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2019 AMC 10B Problem 2

Below is the professionally curated solution for Problem 2 of the 2019 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10B solutions, or check the answer key.

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Concepts:counterexample prime

Difficulty rating: 870

2.

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?

$11$

$15$

$19$

$21$

$27$

Solution:

We need $n$ to not be prime, so $n$ can only be $15,21,27.$ Then, $n-2$ must be not prime, leaving just $27.$

Thus, the answer is E .

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Problem 2 in Other Years

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