A sample of 100 service times at a call center has a sample mean of 9 minutes and a sample standard

deviation of 6 minutes. Assume that the service times are independent and have a normal distribution.

(a) Give a 95% condence interval for the mean service time.

(b) Approximately how many service times we would have to collect to return a 95% condence

interval whose width is at most 15 seconds (= 1/4 minutes)?