How do competing airline carrier fares compare? A frequent flyer investigated round-trip fares for nonstop travel from Philadelphia to 10 different U.S. destinations on Southwest Airlines and US Airways. Data was collected and stored the data in Airlines. (Data extracted from bit.ly/1fIUhec and bit.ly/1eLoDwn.)

a. At the 0.05 level of significance, is there evidence of a difference in the mean round-trip fare between Southwest Airlines and US Airways?

b. What assumption is necessary about the population distribution in order to perform this test?

c. Use a graphical method to evaluate the validity of the assumption in (b).