The Activity page lays out how to perform the different calculations. It might be best to solve one of the problems for the students as a class before having them do the other three on their own. The activity also asks them to compare costs. This requires them to check prices, either in person, on the phone, or on the Net. If this activity has taught us one thing, it's the value of things and how those values change.
Products change, technologies change, tastes change.
All of these things have an effect on prices and on our wallets. Real value comes from how hard we work to get something and how much we appreciate what we have.