The index of refraction of a transparent substance is 1.7, and the speed of light in air is 3.00 · 108 s. calculate the speed of light in the transparent substance.

Given the index of refraction, n and speed of light in the vacuum, c, we can solve for the speed of light in the transparent substance by the equation below.
                                                n = c / v
where v is our unknown.
Substituting the known values,
                                            1.7 = (3 x 10^8 m/s) / v
The value of v is equal to 1.76 x 10^8 m/s. 

It really doesn't matter for any of the questions that you've listed.