In vacuum, to travel distance d, light takes time t and in medium to travel distance 5d, it takes time T. The critical angle of the medium is :

${\mathrm{i}}_{\mathrm{c}}={\sin }^{-1}\left(\frac{1}{\mu }\right)$

Where, μ is the refractive index of the medium w.r.t. vacuum. 

Definition of refractive index is:

$\mu =\frac{c}{v}=\frac{d/t}{5d/T}=\frac{T}{5t}$

On substituting 

$i_c= {\sin }^{-1}\left(\frac{5t}{T}\right)$