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UKMT Intermediate Mathematical Challenge, 2021

Number theory Difficulty 1.0 Junior Find the answer United Kingdom

Kai has begun to list, in ascending order, the positive integers which are $\textit{not}$ factors of 240. What is the sixth number on Kai's list?

(A) $11$ (B) $13$ (C) $14$ (D) $15$ (E) $17$

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