The formula for calculating the index of refraction is n = c/v, where c is the speed of light in a vacuum (3.0 x 10^8 m/s) and v is the speed of light in the medium. The speed of light in the material you're asking about is 1.5 x 10^8 m/s, so the index of refraction can be found by dividing the speed of light in a vacuum by the speed of light in the material. This gives us n = 3.0 x 10^8 m/s / 1.5 x 10^8 m/s = 2.0. So the index of refraction of the material is 2.0.