I had a look at the text, it continues on from Question 4 which has the key piece of information about the electron beam that's directed onto the 'reflecting surface'.
So the electrons in the beam have the same energy as a photon of yellow light -->
a) need to figure out the momentum first before we can do c)
c) determine the number of electrons that are incident on the surface each second in order to produce a pressure of 0.553N m^-2
So Pressure is the force per sq. metre.
The area is
So the force acting on all the electrons to achieve that pressure is:
We also know Newton's second law:
If we consider the force that would act for 1 second, and the momentum that would act for 1 second, and assume that each electron feels the same amount of force, pressure etc, then we can say:
(where k is just a number representing the number of electrons)
electrons.
I think that's the correct reasoning.
edit: hmm, was that what you did as well? I just took a look at what the book had and they had
.
edit: fixed a typo where I had an extra 'k' in the equation when it shouldn't have been there.