Here I 've a problem related to capacitance and its energy. The question goes like this :-
A parallel plate capacitor having plate area 400 cm square. And separation between the plates 1 mm is connected to a power supply of 100 V. A dielectric slab of thickness .5 mm and dielectric constant 5 is inserted into the gap. Find the increase in electrostatic energy.

W = \frac{1}{2}CV^2 = \frac{1}{2} \epsilon \frac{A}{d}V^2

Using this you can work out the difference between the Work stored with and without the insulator.