Because the ball rebounds up then falls back down again before rebounding a next time, I know I can't use the formula for sum of an infinte geometric series, a/(1-r). I'm not sure how to account for the rebounding in this problem.

I realized that the the solution to the question (70.4 ft) is equal to (11/8)(a/(1-r)), but I'm not sure where the 11/8 came from.

Any help is appreciated.