Potential power of the ball at 100 m = kinetic energy at the bottom
The Kinetic energy is = mgH = (20/1000) x 10 x 100 = 20 J.
From the law of convertion we can say 30 % the initial kinetic energy is converted into potential energy at that point as kinetic energy diereses to 70 % of the initial value.
So, assume the point at height as H
(KE = 20)