CEMC Pascal, 2011

Combinatorics Difficulty 3.0 AMC 10/12 Find the answer Canada

Each of the integers 1 to 7 is to be written, one in each circle in the diagram. The sum of the three integers in any straight line is to be the same. In how many different ways can the centre circle be filled?

(A) $1$ (B) $2$ (C) $3$ (D) $4$ (E) $5$

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