Answer
$F = 1.7\times 10^{-13}~N$
Work Step by Step
Let $r = 20~m$
Let $R = 2.0~cm$
We can find the force on the sphere due to the light:
$F = \frac{I~A}{c}$
$F = \frac{P~\pi~R^2}{4\pi~r^2~c}$
$F = \frac{(200~W)~(2.0\times 10^{-2}~m)^2}{(4)~(20~m)^2~(3.0\times 10^8~m/s)}$
$F = 1.7\times 10^{-13}~N$
