The laws of refraction of light are-
i) The incident ray, refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane
ii)  the ratio of sine of angle of incidence to the sine of angle of refraction is a constant , for the light of a given colour and  for the given pair of media

 n_{21} = v_{1} /  v_{2}
 v_{2} = 3* 10^8/1.5=2* 10^8m/s
therefore the speed of light in the given medium is 2* 10^8 m/s