In a plane electromagnetic wave, the electric field oscillates with amplitude  $20V{m}^{-1}$, then the energy density of magnetic field is (in $J/m^3$ )

Average energy density of magnetic field  $=\frac{B_{rms}^2}{2{\mu }_0}=\frac{{(B_0/\sqrt{2})}^2}{2{\mu }_0}=\frac{1}{4}\frac{B_0^2}{{\mu }_0}$
But  $B_0=\frac{E_0}{C}$
Average energy density of electric field=  $\frac{1}{4}\times \frac{E_0^2}{C^2{\mu }_0}$
But    $$\begin{gathered} C=\frac{1}{\sqrt{{\mu }_0{\in }_0}} \\ \therefore {\in }_0=\frac{1}{C^2{\mu }_0} \\ Hence, \\ Average energy density of magnetic field = \frac{1}{4}\times {\in }_0{E}_0^2=\frac{1}{4}\times 8.85\times {10}^{-12}\times {\left(20\right)}^2 \end{gathered}$$
                                                                                                              = $8.85\times {10}^{-10}J/m^3$