Q: A solid sphere of radius R has moment of inertia I about its geometrical axis. It is melted into a disc of radius r and thickness t. If its moment of inertia about the tangential axis (which is perpendicular to plane of the disc), is also equal to I, then the value of r is equal to

(a) $\displaystyle \frac{2}{\sqrt{15}}R $

(b) $ \displaystyle \frac{2}{{\sqrt 5}}R $

(c) $ \displaystyle \frac{3}{\sqrt{15}}R $

(d) $ \displaystyle \frac{\sqrt3}{{\sqrt15}}R $

Ans: (a)