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UKMT Senior Mathematical Challenge, 2017

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

The diagram shows a square and a regular decagon that share an edge. One side of the square is extended to meet an extended side of the decagon. What is the value of $x$ ?

(A) $15$ (B) $18$ (C) $21$ (D) $24$ (E) $27$

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