Energy released per fission $$=200MeV$$
Efficiency of utilization $$=25$$%
Electrical energy produced per fission
$$=\cfrac{25}{100}\times 200=50MeV$$
$$=50\times 1.6\times {10}^{-13}=8\times {10}^{-12}J$$
Targe of electric power $$=200,000MW$$
Electric power to be obtained from nuclear plants
$$\cfrac{10}{100}\times 2000000=20,000MW=2\times {10}^{10}J{s}^{-1}$$
Number of fission reactions required per year
$$=\cfrac{{10}^{10}\times 60\times 60\times 24\times 365}{8\times {10}^{-12}}=3.942\times {10}^{28}$$
Mass of $$^{ 235 }{ U }$$ needed per year
$$=\cfrac{235}{6.023\times {10}^{23}}\times 7.884\times {10}^{28}$$
$$=3.076\times {10}^{7}g=3.076\times {10}^{4}kg$$