Two parallel plate capacitors X and Y have the same area of plates and same separation between them

Two parallel plate capacitors X and Y have the same area of plates and same separation between them. X has air between the plates while Y contains a dielectric medium

(i) Calculate capacitance of each capacitor if equivalent capacitance of the combination is 4 pF.
(ii) Calculate the potential difference between the plates of X and Y.
(iii) Estimate the ratio of electrostatic energy stored in X and Y