U.S. consumers are increasingly viewing debit cards as a convenient substitute for cash and checks. The average amount spent annually on a debit card is $7,800 Assume that this average was based on a sample of 100 consumers and that the population standard deviation is $400. a. At 99% confidence, what is the margin of error?   b. Construct the 99% confidence interval for the population mean amount spent annually on a debit card.

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