\(200 W =200\,J s ^{-1} \quad \lambda=200\,nm =2 \times 10^{-7}\,m\)
\(\therefore \frac{\text { nhc }}{\lambda}=200\)
\(\therefore n =\frac{200 \lambda}{ hc }=\frac{200 \times 2 \times 10^{-7}}{6.6 \times 10^{-34} \times 3 \times 10^8}=2.012 \times 10^{20}\)