A beer distributor claims that a new display, featuring a life-size picture of a well-known athlete, will increase product sales in supermarkets by an average of 40 cases a week. For a random sample of 25 supermarkets, the average sales increase was 31.3 cases and the sample standard deviation was 12.2 cases. Test, at the 5 percent level, the null hypothesis that the population mean sales increase is at least 40 cases, stating any assumption you make.