Lag1989 problem number 14.22

Inside the flat capacitor there is a plate parallel to the plates, the thickness of which is equal to 0.6 of the gap between the plates. The capacitance of the capacitor without a plate is C = 20 nF. The capacitor was first connected to a constant voltage source U = 200 V, then it was disconnected, and after that the plate was slowly removed from the gap. Find work A, spent on removing the plate, if the plate is: a) metal; b) glass.