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. A previous study indicated that the standard deviation for long-distance 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?