$I=\frac{P}{4\pi {{d}^{2}}}=\frac{30}{4\pi \times {{5}^{2}}}=\frac{3}{10\pi }\frac{W}{{{m}^{2}}}$ $I=\frac{E}{A\times \Delta t}\Rightarrow \frac{3}{10\pi }=\frac{E}{(\pi \times {{10}^{-4}})\times 300}\Rightarrow E=9\times {{10}^{-3}}J$ $E=n\frac{hc}{\lambda }\Rightarrow 9\times {{10}^{-3}}=n\times \frac{6/6\times {{10}^{-34}}\times 3\times {{10}^{8}}}{6/6\times {{10}^{-7}}}\Rightarrow n=3\times {{10}^{16}}$