Question

the absolute refractive indices of glass and water are 4/3 and 3/2 respectively.If the speed of light in glass is 2X10^8 m/s, calculate the speed of light in
(i)vacuum
(ii)water

Answer 1

speed of light changes according to the medium it is in.
in vacuum, speed of light is always  3x10^8 ms^ -1
formula required here is n= c/v 
where n is refractive index
c is speed of light in vacuum,
v is speed of light in medium.
substitute the values n find the answer.

Answer 2

I hope you have given the data of refractive indexes correctly.  Normally water has 4/3 and  glass = 3/2.  You have given the other way round.

$\mu_g=4/3,\ \ \ \mu_w=3/2\\\\c_{gl}=2*10^8m/s,\ \ \ c_w=?,\ c=?\\\\\mu_g=\frac{c}{c_{g}}\\\\In\ vacuum\ c=\mu_g*c_g=\frac{4}{3}*2*10^8=2.667*10^8 m/s\\\\c_w=\frac{c}{\mu_w}=\frac{2.667*10^8}{3/2}=1.778*10^8m/s$