$\text{Velocity in air } v_a = c$

$\text{Velocity in medium } v_m = \frac{c}{2}$

$\Rightarrow \frac{v_a}{v_m} = 2$

$\text{Since mediums are non magnetic } \Rightarrow \mu_a = \mu_m = 1$

$\Rightarrow \frac{1/\sqrt \epsilon_a}{1/\sqrt \epsilon_m} = 2$

$\Rightarrow \frac{\epsilon_a}{\epsilon_m} = \frac{1}{4}$