$I=\frac{P\text{of source}}{4\mathrm{\pi }{\left(\text{distance}\right)}^2}=\frac{P}{4\pi d^2}$
Here, we assume light to spread uniformly in all directions.
The number of photo-electrons emitted from a surface depends on the intensity of light $I$ falling on it. Thus the number of electrons emitted $n$ depends directly on $I$. $P$ remains constant as the source is the same.
 
$\therefore$   $\genfrac{}{}{0.1ex}{}{I_2}{I_1}=\genfrac{}{}{0.1ex}{}{n_2}{n_1}\Rightarrow \genfrac{}{}{0.1ex}{}{P_2}{P_1}{\left(\genfrac{}{}{0.1ex}{}{d_1}{d_2}\right)}^2=\genfrac{}{}{0.1ex}{}{n_2}{n_1}$
 
$\therefore$   $\genfrac{}{}{0.1ex}{}{n_2}{n_1}=\left(\genfrac{}{}{0.1ex}{}{P}{P}\right){\left(\genfrac{}{}{0.1ex}{}{1}{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}\right)}^2=\genfrac{}{}{0.1ex}{}{4}{1}$     $(\because P_1 =P_2 =P)$