Brian Vanecek, VP of Operations at Portland Trust Bank, is evaluating the service level provided to walk-in customers. Accordingly, his staff recorded the waiting times for 64 randomly selected walk-in customers and determined that their mean waiting time was 15 minutes. Assume that the population standard deviation is 4 minutes. The 90% confidence interval for the population mean of waiting times is ________. (Points : 4) 14.27 to 15.73 14.18 to 15.82 9.88 to 20.12 13.86 to 16.14 18.12 to 19.87