You randomly select 20 cars that were sold at a dealership and determine the number of days each car sat on the dealership's lot before it was sold. The sample mean is 9.75 days, with a sample standard deviation of 2.39 days . Fill in the blank below that represents a 90% confidence interval for the population mean number of days the car sits on the dealership's lot . Assume the days on the lot are normally distributed .