The moment of inertia of a solid sphere is $$\frac{2}{5}MR^{2}$$

The moment of inertia of a disc is $$\frac{1}{2}MR^{2}$$

The moment of inertia of a disc through its edge is {1}{2}MR^{2} + MR^{2}\)

As both moment of inertia are equal

$$\Rightarrow {2}{5}MR^{2} + {3}{2}MR^{2}$$

Therefore,$$r = \frac{2R}{\sqrt{15}}$$

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