Question: Please construct a confidence interval in the situation described below. You need to choose which Excel Worksheet to use (CI Sigma_Known.xls or CI_Sigma_Unknown.xls) and ex­plain your choice.

A major department store chain is interested in estimating the average amount its credit card cus­tomers spent on their first visit to the chain's new store. 15 credit card accounts were randomly sampled and analyzed with the following results: mean is $50.50 and S² is 400. Construct a 95% confidence interval for the average amount its credit card customers spent on their first visit to the chain's new store in the mall assuming that the amount spent follows a normal distribution.