if $2.5\times {10}^{-6}N$ average  force  is exerted  by a  light  wave  on a  non –reflecting  surface  of $30c{m}^2$ area  during  40 minutes  of  time span the energy flux  of  light  just before  it  falls on the surface  is --- $w/c{m}^2$ (round off  to the  nearest integer ) (assume complete absorption  and normal absorption  and  normal incidence conditions are  there )