A survey is planned to determine the mean annual family

A survey is planned to determine the mean annual family medical expenses of employees of a large company. The management of the company wishes to be 95% confident that the sample mean is correct to within ± $50 of the population mean annual family medical expenses. A previous study indicates that the standard deviation is approximately $ 400.,a. How large a sample is necessary?,b. If management wants to be correct to within ± $25, how many employees need to be selected?