homework stat

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 a 95% confidence interval and a margin of error of no more than 3.75 minutes. a. A previous study indicated that the standard deviation for long-distance calls is 25 minutes per month. What should the sample size be? b. Determine the required size if the confidence level were changed from 95% to 90%. c. What would the required sample size be if the confidence level was 90% and the margin of error was 6 minutes