according to a leasing firm's reports, the mean number of miles driven annually in its leased cars is 12940 miles with a standard deviation of 2920 miles. the company recently starting using new contracts which require customers to have the cars serviced at their own expense. the company's owner believes the mean number of miles driven annually under the new contracts, u, is less than 12940 miles. he takes a random sample of 50 cars under the new contracts. the cars in the sample had a