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UKMT Intermediate Mathematical Challenge, 2012

Algebra Difficulty 1.0 Junior Find the answer United Kingdom

What value of $x$ makes the mean of the first three numbers in this list equal to the mean of the last four?
$15 \qquad 5 \qquad x \qquad 7 \qquad 9 \qquad 17$

(A) $19$ (B) $21$ (C) $24$ (D) $25$ (E) $27$

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