A parallel plate capacitor with width $4 \mathrm{cm}$, length $8 \mathrm{cm}$ and separation between the plates of $4 \mathrm{mm}$ is connected to a battery of $20 \mathrm{V}$. A dielectric slab of dielectric constant $5$ having length $1 \mathrm{cm}$, width $4 \mathrm{cm}$ and thickness $4 \mathrm{mm}$ is inserted between the plates of parallel plate capacitor. The electrostatic energy of this system will be _____ ${ϵ}_0 \mathrm{J}$. (Where ${ϵ}_0$ is the permittivity of free space)

Hence, the correct answer is 240.