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UKMT Pink Kangaroo, 2016

Combinatorics Difficulty 3.0 AMC 10/12 Find the answer United Kingdom

Peter wants to colour the cells of a $3 \times 3$ square in such a way that each of the rows, each of the columns and both diagonals have cells of three different colours. What is the least number of colours Peter could use?

(A) $3$ (B) $4$ (C) $5$ (D) $6$ (E) $7$

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