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UKMT Intermediate Mathematical Challenge, 2016

Geometry Difficulty 2.0 Junior Find the answer United Kingdom

The shaded region in the diagram, bounded by two concentric circles, is called an annulus. The circles have radii 2 cm and 14 cm. The dashed circle divides the area of this annulus into two equal areas. What is its radius?

(A) $9\text{ cm}$ (B) $10\text{ cm}$ (C) $11\text{ cm}$ (D) $12\text{ cm}$ (E) $13\text{ cm}$

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