A study addressed the per-person spending at selected airports for merchandise, excluding food, gifts, and news items. A file titled Revenues contains sample data selected from airport retailers in 2012 and again in 2015.

a. Conduct a hypothesis test to determine if the average amount of retail spending by air travelers increased at least as much as approximately $0.10 a year from 2012 to 2015. Use a significance level of 0.025.

b. Using the appropriate analysis (that of part a or other appropriate methodology), substantiate the statement that average retail purchases in airports increased over the time period between 2012 and 2015. Support your assertions.

c. Parts a and b give what seems to be a mixed message. Is there a way to determine what values are plausible for the difference between the average revenues in 2012 and 2015? If so, conduct the appropriate procedure.