clear
//Given
f=18                         //W/cm**2
A=20                          //cm**2
t=30*60
c=3.0*10**8

//Calculation
U=f*A*t
P=U/c
F=P/t
P1=2*P
F1=P1/t

//Result
printf("\n Average force exerted on the surface is %0.3f  N", F1)