Explanation

(b) The angle subtended by the image of the sun at the mirror
$= 30^{'} = \left( \frac{1}{2} \right)^{o} = \frac{\pi}{360}rad$
If $x$ be the diameter of the image of the sun, then
$\frac{Arc}{Radius} = \frac{x}{100} = \frac{\pi}{360}$
$\Rightarrow x = \frac{100\pi}{360} = 0.87\ cm$