A point source of light is placed at the centre of curvature of a...

For completely reflecting surface, force, $F = \dfrac{2IA}{c}$, where, $I = \dfrac{P}{Surfacearea}$ is intensity of radiation, $A$ is surface area and $c$ is speed of light.

$F = \dfrac{2I}{c}\pi{R}^{2} = 2\dfrac{{P}_{0}}{4\pi{R}^{2}}\dfrac{\pi{R}^{2}}{c}$

$= \dfrac{{P}_{0}}{2c} = \dfrac{24}{2 \times 3 \times {10}^{8}} = 4 \times {10}^{- 8}N$