A 67.5kg skier slides down a 20° slope whose vertical height is 125m. What is the variation of the potential energy of the skier?

Answer 1

well it gives you the height, not the length, so you do not need the 20 degrees
- m g h = -67.5 * 9.81 * 125 Joules of gravitational potential energy lost.

By the way
if there is no friction, that is (1/2) m v^2 the increase in kinetic energy.

Now if you need to calculate friction forces, you need to use the 20 degrees.