Everley's syndrome is a rare congenital disease that generally causes a reduction in the concentration of blood sodium. This is thought to provide a useful diagnostic sign as well as clue to the efficacy of treatment. Little is known about the subject, but the director of a dermatological department in a London teaching hospital is known to be interested in the disease and has seen more cases than anyone else. The blood sodium concentrations of the 18 patients he has seen have a distribution that appears to be bell shaped with a mean of 115mmo/L and a standard deviation of 12mmol/L. You need to construct a 95% confidence interval for the mean of all such blood sodium concentrations. To help you do so, answer each of the following questions.