The speed of light in a certain substance is 83% of its value in water. what is the index of refraction of this substance?

The Index of Refraction formula is:
n = c / v,
where c is the speed of light in vacuum and v is the speed of light in that substance.
For water: n (water) = 1.33
1.33 = c / v(water)
v(water) = c/1.33
For a certain substance:
n = c : (0.83 * c/1.33) = 1.33 / 0.83
n = 1.6
The Index of Refraction of this substance is 1.6.  

By dividing you can find the answer