To find the bounce efficiency of the tennis ball, we must first find the amount of potential energy it has at it maximum height after being hit, and at its maximum height after the first bounce. To do this we can use our potential energy equation, which involves mass, acceleration due to gravity, and height. Once we know the amount of energy at both these points, we can put them into our efficiency equation, and find how efficiently the ball bounces:
Therefore the bounce efficiency of the ball is 70%.