Given the following information about the speed of light:
Speed of light in vacuum which has a refractive index of 1 is 300Km/sec
Speed of light in water which has a refractive index of 1.3 is 225Km/sec
Speed of light in diamond which has a refractive index of 2.4 is 125Km/sec

Estimate the speed of light in glass which has a refractive index of 1.5
From the information given we can see the light of speed is inversely proportional the refractive index of the medium. We can estimate the speed of light in glass as speed of light in vacuum divided by the medium's refractive index. In case of glass that would be 300Km/1.5 = 200Km/sec.

The estimated speed of light in glass will be around 200 Km/sec