A bubble in glass slab $(\mu = 1.5)$ when viewed from one side appears at $5 \ cm$ and $2 \ cm$ from other side, then thickness of slab is :

Acco. To given information. [ #] i use this for refractive index we know, refractive index = real depth/ apparent depth 

case 1. x/5=1.5 and case 2 x/2= 1.5 now add both case we get x = 10.5 as answer