7.28 Health care costs.

The cost of health care is the subject of many studies that use statistical methods. One such study estimated that the average length of service for home health care among people aged 65 and over who use this type of service is 242 days with a standard error of 21.1 days. Assuming sample size larger than 1000, calculate a 90% confidence interval for the mean length of service for all users of home health care aged 65 and over.¹³