Light problems?

Question

Light is incident normally on a 1cm. layer of water that lies on top of a flat Lucite plate with a thickness of 0.5cm. How much more time is required for light to pass through this double layer than is required to traverse the same distance in air? ( n lucite is 1.59)

2. Find the speed of light in a. flint glass, water and zircon.

Answer 1

v=c/n and t=d/v so t=dn/c