The mean life of a random sample of 20 soccer balls of a certain brand is 752 days. (a) If the history in usage of that kind of ball shows the standard deviation of 56 days, what is the two-sided 95% confidence interval on the mean? (b) If the variance of the history of the usage of the ball is not known and the standard deviation of our random sample is 45 days, what is the two-sided 90% confidence interval?
a) (705.22 days, 798.78 days); (728.89 days, 775.11 days)
b) (734.16 days, 769.84 days); (701.43 days, 802.57 days)
