Delivery time by courier services is important to stock brokers. One such firm wishes to estimate the average time taken by couriers delivering negotiable securities in New York City. Specifically, the firm randomly samples 36 courier pickup and delivery times. The collected data, in minutes, represent the time between pickup and delivery. Suppose the population standard deviation is 0.3 minutes and the sample mean is 9.975. Find the sample mean and compute. A 90-percent confidence interval for the true mean delivery time, is: