A parallel plate capacitor with width 4   cm , length 8   cm and separation between the plates of…

The figure below shows the parallel plate capacitor with dielectric slab placed in between the plates.

Now, the effective capacitance of the combination is 

${\mathrm{C}}_{\mathrm{eff}}=\left[\frac{{\epsilon }_0\left(7\times 4\right)}{{\displaystyle \frac{4}{10}}}+\frac{5{\epsilon }_0\left(1\times 4\right)}{{\displaystyle \frac{4}{10}}}\right]\times {10}^{-2}$

$=\left[70{\epsilon }_0+50{\epsilon }_0\right]\times {10}^{-2}=120{\epsilon }_0\times {10}^{-2}$

$\Rightarrow C_{\mathrm{eff} }=1.2{\epsilon }_0$

Electrostatics energy of this system is $=\frac{1}{2}{C}_{\mathrm{eff}}{V}^2$

$=\frac{1}{2}\left(1.2\right){\epsilon }_0\left(20\right)\left(20\right)=240{\epsilon }_0$