On a particular day a consumer-advice bureau received 125 calls. For a random sample of 40 of these calls, it was found that mean time taken in providing the requested advice was 7.28 minutes, and the sample standard deviation was 5.32 minutes.
a. Find a 99% confidence interval for the mean time taken per call.
b. Find a 90% confidence interval for the total amount of time taken in answering these 125 calls.