Question: Amazon.com claims that its textbook prices are cheaper than those offered by the local college bookstore. To test this claim, you randomly select 15 textbooks that are sold by both. Data File TEXTBOOK gives the prices for those 15 randomly selected textbooks at the local college bookstore and at Amazon.com.

a. Construct a 95% confidence interval estimate for the mean selling price of textbooks at the local bookstore.

b. Construct a 95% confidence interval estimate for the mean selling price of textbooks at Amazon.com.

c. Do the results of parts (a) and (b) support the claim made by Amazon? Explain your answer.

d. What must be true about the data sets to make the confidence intervals you constructed in parts (a) and (b) valid?