A point source of radiation of power P is placed on the axis of completely absorbing disc. The distance between the source and the disc is 2 times the radius of the disc. The force that light exerts on the disc is $\frac{P}{nc}$ . Then, the value of  n  is

Number of photons striking per second  $=N=\frac{IA}{hv}$
Consider a ring of radius $x$  and width $dx$  on the disc.
Intensity on the ring due to source is 
 $I=\frac{P}{4\pi r^2}$
 $dA=2\pi xdx$

On a ring, only $F\cos \theta$  component will remain $F\sin \theta$ and will cancel out$\int dF=\int \left(\frac{Id{A}_{\perp }}{hv}\right)\left(\frac{h}{\lambda }\right)\cos \theta$
 $\Rightarrow F={\int }_0^R\left[\frac{P}{4\pi (4R^2+x^2)}\right]\left[\frac{2\pi xdx\cos \theta }{\left(\frac{hc}{\lambda }\right)}\right]\left(\frac{h}{\lambda }\right)\cos \theta$
$\Rightarrow F=\frac{P}{20c}$

So,  $n=20$