In a medium the speed of light wave decreases to 0.2 times to its speed in free space. The ratio of relative permittivity to the refractive index of the medium is x :1. The value of x is ____.
( Given speed of light in free space $=3\times {10}^8{\mathrm{ms}}^{-1}$ and for the given medium ${\mu }_r=1$)