A population is normal with a variance of 9. Suppose you wish to estimate the population mean. Find the sample size needed to assure with 90% confidence that the samnple mean will not differ from the population by mnore than 2 units.
A sample of 25 different payroll departments found that the employees worked an average of 310.3 days a year with a standard deviation of 23.8 days. What is the 90% confidence interval for the average days worked in all payroll departments? Assume the number of days worked is normally distrributed.