How do the fares for airlines that directly compete compare? The file Airlines contains roundtrip fares for nonstop travel from Philadelphia to 10 different U.S. destinations on Southwest Airlines and American Airlines that a frequent flyer collected.

Source: Data extracted from www.southwest.com/ and www.aa.com/.

a. At the 0.05 level of significance, is there evidence of a difference in mean roundtrip fare between Southwest Airlines and American Airlines?

b. What assumption is necessary in order to perform this test?

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