A ray of light ab passing through air, enters a liquid of refractive index  ${\mu }_1=1.2$, at the boundary XY. In the liquid, the ray is shown as bc. The angle between ab and bc  in degree is  ${36}^0$  (angle of deviation). The ray then passes through a rectangular slab ABCD of refractive index  ${\mu }_2=1.5({\mu }_2>{\mu }_1)$, and emerges from the slab as ray de. The angle between XY and AB is  ${30}^0$. The angle (in degree) between ab and de is  $4x$. Find the value of  x.