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UKMT Junior Mathematical Challenge, 2020

Combinatorics Difficulty 1.0 Junior Find the answer United Kingdom

Wesley has a grid of six cells. He wants to colour two of the cells black so that the two black cells share a vertex but not a side. In how many ways can he achieve this?

(A) $2$ (B) $3$ (C) $4$ (D) $5$ (E) $6$

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