What is the time taken (in seconds ) to cross a glass of thickness 4 mm and `mu= 3` by light

The time taken by light to cross a glass slab of thickness 4 mm and refractive index 3 is

The time taken by the light to cross a glass of thickness 4 mm and refractive index (mu = 3) , will be

What is the time taken by light to travel through a glass slab of thickness ‘d’ and refractive index mu ?

How much time will sunlight take to pass through the glass window of thickness 4 mm ? mu of glass = 1.5.

The time taken by a ray of light to travel through a glass slab of thickness 2 cm and refractive index 1.5 will be