A hotel manager would like to know whether customers who pay by different methods have different-sized bills. She divides all customers into four categories: those who pay by check or cash, those who pay with a VISA or MasterCard, those who pay with an American Express card, and those who use some other type of credit card. She then collects data on daily bills, which are given in the file P12_4.XLS. Note that these bills contain the room charge, plus any other charges to the customer’s account. 

a. Test whether the different categories of customers have different-sized bills at the 10% significance level. 

b. Compute 90% confidence intervals for all pairs of differences between means. Which of these differences, if any, are significantly nonzero at the 10% significance level?