The period of revolution of a satellite is 90min. What is the altitude above the surface of the earth?
Quite low altitude. The period would be 84 minutes if it could be in a sea-level orbit.
The earth's radius is Re = 6378 km
Centripetal force must equal gravity attraction
V^2/R = G*M/R^2
V = 2 pi R/(period)
Eliminate V and solve for R. G is the universal gravity constant.
The altitude is R - Re.