An independent group of food service personnel conducted a survey on tipping practices in a large metropolitan area

An independent group of food service personnel conducted a survey on tipping practices in a large metropolitan area. They collected information on the percentage of the bill left as a tip for 40 randomly selected bills. The average tip was 12.1% of the bill with a standard deviation of 2.6%. Assume that the tips are approximately normally distributed. Construct an interval to estimate the true average tip (as a percent of the bill) with 90% confidence. Round the endpoints to two decimal places, if necessary.