Two monochromatic beams A and B of equal intensity I, hit a screen. The number of photons hitting the screen by beam A is twice that by beam B. Then what inference can you make about their frequencies?

The two monochromatic beams are of same Intensity. The Intensity is given by

Considering they both are incident on the same area of cross-section (on the same screen), we now have to deal with only the power of the beams. Since, they have the same intensities, they will have the same power and in turn same energy per second. (E_A for beam A and E_B for beam B)

Here, N_A and N_B is the number of photons emitted by beam A and B respectively and and are the energies of one photon of beam A and B respectively.

According to the question, N_A is two times N_B

Thus, the frequency of beam B is two times that of beam A.