The owner of an oil change shop would like to know the mean time it takes to do an oil change at the shop. He takes a random sample of 16 cars that were serviced and finds a sample average of 20.5 minutes and a standard deviation of 7.4 minutes.
a) Construct a 90% confidence interval for the true mean time to do an oil change to a car.
b) Interpret the confidence interval.
c) Are you assuming anything that is not directly given to you in the problem to construct the confidence interval in 9.41a? Explain.