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UKMT Grey Kangaroo, 2022

Combinatorics Difficulty 2.0 Junior Find the answer United Kingdom

Jenny decided to enter numbers into the cells of a $3 \times 3$ table so that the sum of the numbers in all four possible $2 \times 2$ cells will be the same. The numbers in three of the corner cells have already been written, as shown. Which number should she write in the fourth corner cell?

(A) $0$ (B) $1$ (C) $4$ (D) $5$ (E) $6$

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