Thew energy stored in a capacitor of capacitance [tex] C [/tex] and voltage between the plates [tex] V [/tex] is
[tex] E=\frac{1}{2} CV^2=\frac{1}{2C} Q^2 [/tex].
Substituting numerical value
[tex] E=\frac{1}{2*20*10^{-6}} (60*10^{-6})^2\\
E=90\; \mu J [/tex]
