Light travels a distance $x$ in time $t_1$ in air and $10x$ in time $t_2$ in another denser medium. What is the critical angle for this medium?
$v_{air} = x/t_1$, $v_{med} = 10x/t_2$; $n = v_{air}/v_{med} = t_2/(10t_1)$. $\sin\theta_c = 1/n = 10t_1/t_2$.