The energy used in the process is used to rebound the ball to a certain height. The efficiency of the first rebound is equal 1.15/1.50 or 0.77. Hence the not used is the equal to the other part of the fraction where ball did not fully rebound to the original height. In this case, the equation is (1-0.77)^x = 1 - 0.93 where x is the number of rebounds until 93% of the energy is lost. x is equal to approx 10 rebounds.
