Hello Student,
In the given question, the energy in a single photon =hv = hc/λ
= 6.626 x 10-34 x 3 x 108 / 0.57 x 10-6
= 3.4874 x 10-19 J
Now, Quanta(photon) per second will be obtained as
Energy emitted per second(or power) by bulb / energy of a single photon
= 25 Js-1 / 3.4874 x 10-19 J
= 7.17 x 1019 s-1
Hope it helps.
Regards,
Kushagra