We know that the Refractive index = real depth/apparent depth = 1.5

Let the thickness of the glass slab be x

real depth is x cm.

so apparent depth = (x-4) cm.

real depth = 1.5 * apparent depth

x = 1.5*(x-4)

0.5x = 6

x = 6/0.5 = 12

**So, Thickness of glass is 12 cm**