Power at the distance \(r\) from source, \(P=\frac{P_0}{4 \pi r^2}\)
Each photon emits one electrons.
Also, \(P \propto N\)
\(N\) is the number of photons
\(N \propto \frac{1}{r^2}\) \(\Rightarrow \frac{N_2}{N_1}=\frac{r_1^2}{r_2^2}\)
\(\frac{N_2}{N_1}=\frac{0.5^2}{1^2}=\frac{1}{4}\)