Answer
$61photons/s$
Work Step by Step
We can find the required number of photons per second as
$n=\frac{I(\pi R^2)}{hf}$
We plug in the known values to obtain:
$n=\frac{(5.0\times 10^{-13})(3.14)(4.25\times 10^{-3})^2}{(6.63\times 10^{-34})(7.0\times 10^{14})}$
$n=61photons/s$
