Galileo used inclined planes to investigate "freefall."
why did he do that instead of experimenting with velocitydropped
objects?

Because of the reason that the rate at which the velocity dropped object fall depends on their mass and may vary.

Explanation:

Galileo's investigation of free fall of objects using inclined planes led him to measure acceleration accurately and to conclude that all objects fall at the same rate irrespective of their masses under the action of gravity when the external forces on the object are absent.

Whereas as far as the velocity dropped objects are concerned they acquire different velocities for variation in masses.

Thus the velocity dropped objects may drop or fall at different rates and not at the same rate.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.