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Answer: $\frac12$
The radius of the semicircle is twice the radius of the circle
Area $=\frac12\left(\pi\left(2r\right)^2\right) = \frac12\left(\pi\times4r^2\right) = 2\pi r^2$
Area $=\pi r^2$ which is $\frac12$ of $2\pi r^2$
This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.