A plate of mass M remains in equilibrium in air when n bullets are fired per second on it. The mass of each bullet is m and it strikes the plate with speed v' If the coefficient of restitution is e, then (M>>m)

see fig. (4) the change in momentum of Bullet = m v e - (-m v) = m v (1+e)given that n bullets strike per second '. Force exerted by the bullets  =n[mv(1+e)] upwards   Now n[mv(1+e)]=Mg (∵F=Mg downwards ) so, M=nmv(1+e)g