magnetic field hcv doubt

i am stuck in solving the problem

question no 39 in magnetic field of hcv part 2

a particle of mass m and charge q is projected into a region having a perpendicular magnetic field b.find the angle of deviation of the particle, if the width is d

i even saw the soln of the net,there they assumed the width d as the radius of the circular motion.....i am quite confused on that part.

regards,
funkygp