Tennis balls must have a high rebound, or bounce, that is approved by the International Tennis Federation. Rebound is measured by dropping a ball from a height of 100 inches and measuring how high it bounces. Approved balls must bounce back up 53 to 58 inches. A tennis coach tested 50 tennis balls and found that the mean bounce was 56.2 inches and the standard deviation was 1.8 inches The z-score of a ball that bounces 56.2 inches is This ball has a bounce that is the mean.