Speed of light in medium is related to speed of light in vacuum $\left(c\right)$ and refractive index $\left(\mu \right)$ by $v=\frac{c}{\mu }$.

Then, we have $\mu =\frac{c}{v}=\frac{c}{0.2c}$

$\Rightarrow \mu =5$

Refractive index in terms of relative permeability $\left({\mu }_r\right)$and relative permittivity$\left({\epsilon }_r\right)$  of the medium is stated as $\mu =\sqrt{{ϵ}_r {\mu }_r}$

$\Rightarrow {ϵ}_r=\frac{{\mu }^2}{{\mu }_r}$

$\therefore \frac{{ϵ}_r}{\mu }=\frac{\mu }{{\mu }_r}=5$

Hence, the value of $x=5$.