Question

refractive indexof diamond is 2.42 and that of glass is 1.51 then how much does light travel in glass than in diamond

Answer 1

v1 = speed of light in diamond = speed of light in vacuum / 2.42
                                         = c / 2.42
v2 = speed in glass =  c / 1.51

v2 :  v1  =  1/2.42  :  1/1.51    =  1.51  :  2.42    = 62.39 : 100

Answer 2

velocity of light in medium inversely proportional to refractive index of that medium .
so,
velocity of light in diamond/velocity of light in glass = refractive index of glass/refractive index of diamond

=1.51/2.42 =151/242

hence,
ratio is 151 : 242