For the plane electromagnetic wave given by $E=E_0\sin (\omega t–kx)$ and $B=B_0\sin (\omega t-kx),$ the ratio of average electric energy density to average magnetic energy density is 

The formula to calculate the average electric energy density is given by

$U_E=\frac{1}{2}{\epsilon }_0{E}^2 ...\left(1\right)$

and, the formula to calculate average magnetic energy density is given by

$U_B=\frac{1}{2{\mu }_0}{B}^2 ...\left(2\right)$

For any particular electromagnetic wave, both magnetic and electric field are equally involved in contributing to energy density. Hence, the ratio of average electric energy density to magnetic energy density is $1$.