For Exercises 51–58, do the following:
57. Hybrid Vehicles. A study by Edmunds.com examined the time it takes for owners of hybrid vehicles to recoup their additional initial cost through reduced fuel consumption. Suppose that a random sample of nine hybrid cars showed a sample mean time of 2.1 years. Assume that the population is normal with . Test, using level of significance , whether the population mean time it takes owners of hybrid cars to recoup their initial cost is less than three years.
9.3.57
(a) . Reject if the . (b) −13.5 (c) (d) Since the , reject . There is evidence that the population mean time hybrid cars take to recoup their initial cost is less than 3 years.