Initial potential energy in ball = mgh

Where, m – mass of the ball

g- acceleration due to gravity = 10m/sec

h- height of object = 10 m

(since mass of ball is not known we will keep in variable form)

P.E= m× 10 × 10 = 100m Joule

If 40% of energy is reduced, then 60% energy is left

Thus, the energy left =

= 60m Joules

If h’ is the height attained when bouncing back, then

60m = mgh’ = m(10)h’

h’ = 6m