The ratio of average electric energy density and total average...

Average electric energy density is stated as ${u}_{E} = \dfrac{1}{2}{ε}_{0}{E}^{2}$

Total average energy density $U = \dfrac{1}{2}{ε}_{0}{E}^{2} + \dfrac{1}{2{\mu}_{0}}{B}^{2}$

$= \dfrac{1}{2}{ε}_{0}{E}^{2} + \dfrac{1}{2{\mu}_{0}}\dfrac{{E}^{2}}{{c}^{2}}$ (As $B = \dfrac{E}{c}$)

$= \dfrac{1}{2}{ε}_{0}{E}^{2} + \dfrac{1}{2{\mu}_{0}}{E}^{2}{ε}_{0}{\mu}_{0} = \dfrac{1}{2}{ε}_{0}{E}^{2} + \dfrac{1}{2}{E}^{2}{ε}_{0} = {ε}_{0}{E}^{2}$

Hence, the ratio, $\dfrac{{u}_{E}}{U} = \dfrac{1}{2}$.