Let nA represent the number of photons that fall per second from beam A, and nB represent the number of photons that fall per second from beam B.
Given,
A has twice as many photons reaching the screen as B.
Therefore, nA = 2nB
Energy from the beam A = hvA
Energy from the beam B = hvB
Intensity = I = nAvA = nBvB
vB = 2vA