Volume = area*thickness
=> thickness = volume/area
= 0.05/40 = 0.00125 cm = 1 x 10^-3 cm which is one sig fig, which i think is the correct amount to use here. 0.05 has one sig fig, 40 has 2, i believe the rule is the answer should have as many sig fig's as the quantity with the least sig figs. But i may be wrong.
Use your units as a guide if you don't remember formulas. We know diameter has to be in cm. we are given cm^3 and cm^2, the only way to get cm from those is to divide cm^3 by cm^2.