Energy = hc/?
Energy = 6.62 x 10^-34 x 3 x 10^8/ 500 x 10^-9
Energy = 4 x 10^-19J

Power = 60W
N = 60 x 10^19 /4
N = 15 x 10^19

Efficiency is 2% that means only 2% of the calculated N will be emitted.
So final answer is 3 x 10^18

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