Light with an energy flux of 18 watts / $c{m}^2$ falls on a perfectly reflecting surface has an area of $20c{m}^2,$
find the average force exerted on the surface during a 30 minute time span

Energy flux $=18W{\text{ cm}}^{-2}=18{\text{ Js}}^{-1}c{m}^{-2}$
Area =$20 c{m}^2$  
Time $=30 min =1800s$
Total energy falling on the surface 
$$\begin{gathered} u=Energy\text{ flux }\times \text{ time }\times \text{ area} \\ {\text{=(18Js}}^{-1}c{m}^{-2})\times 1800s\times 20c{m}^{\begin{array}{l}2\\ \end{array}} \\ =6.48\times {10}^{5}J \end{gathered}$$ 
the total momentum delivered to the surface,
 $P=\frac{u}{c}=\frac{6.48\times {10}^{5}J}{3\times {10}^{8}m{s}^{-1}}$
$=2.16 \times {10}^{-3}kgm{s}^{-}$
The average force exerted on the surface ,
$F=\frac{P}{t}=\frac{2.16\times {10}^{-3}}{1800}=1.2\times {10}^{-6}N$ 
In case, the surface is a perfect reflector, the change of momentum $=p-(-p)=2p$
And  $F=2\times 1.2\times {10}^{-6}=2.4\times {10}^{-6}N$