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UKMT Senior Mathematical Challenge, 2016

Algebra Difficulty 2.0 Junior Find the answer United Kingdom

In the grid below each of the blank squares and the square marked $X$ are to be filled by the mean of the two numbers in its adjacent squares. Which number should go in the square marked $X$ ?

(A) $15$ (B) $16$ (C) $17$ (D) $18$ (E) $19$

Figure for this problem

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