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UKMT Senior Mathematical Challenge, 2019

Number theory Difficulty 2.0 Junior Find the answer United Kingdom

Which of the following five values of $n$ is a counterexample to the statement in the box below? For a positive integer $n$ , at least one of $6n-1$ and $6n+1$ is prime.

(A) $10$ (B) $19$ (C) $20$ (D) $21$ (E) $30$

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