-- The energy of one photon is <em>(h · frequency of the light)</em>
' h ' is 6.626 × 10⁻³⁴ m²-kg/s ("Planck's Constant")
-- The question doesn't tell you the frequency of the light from the LED, but it tells you the wavelength, and
<em>Frequency = (speed of light) / (wavelength) </em>.
-- Now you have everything you need to calculate the <em>energy carried by one photon from the LED</em>.
-- The power of the light from the LED is 120 milliwatts. That's <em>0.120 Joule of energy per second</em>.
Now you should be able to find the number of photons per second. It's going to be <em>(0.120 Joule) / (energy carried by one photon)</em> .
When I scribbled it out on a scrap of scratch paper, I got 3.853 x 10³⁸ photons, but you'd better really check that out.