Question

refractive index of diamond relative to air is 2.42 and that if glass is 1.51 how much faster does light travels in glass than in diamond?

Answer 1

R = C/V

refractive index in glass $R_{g} = C/ 1.51$

refractive index in diamond $R_{d}$ = C/2.42

they asked how faster does a light travel in glass than in diamond 
so, 

$\frac{ R_{g} }{R _{d} } = \frac{C}{1.51 }$ x $\frac{2.42}{C}$

so R = $\frac{2.42}{1.51}$

R= 1.602604 (approx)