The Northwest Pacific Phone Company wishes to estimate the average number of minutes its customers spend on long-distance calls per month. The company wants the estimate made with 99% confidence and a margin of error of no more than 5 minutes.

a. Assume that the standard deviation for longdistance calls is 21 minutes per month. What should the sample size be?

b. Determine the required sample size if the confidence level were changed from 99% to 90%.

c. What would the required sample size be if the confidence level was 95% and the margin of error was 8 minutes?