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One of my favorite problems for a group to solve is: How many area codes are available for use? I give students the OLD SET of rules for area codes: - can't begin with a 0 or 1 - can't end in a 0 or 1 - middle digit must be a 0 or 1 - must be three digits (Please note that these rules no longer apply!) Cooperatively they work to make an organized list of possibilities. Then a discussion ensues regarding why the phone company found it necessary to change rules. (More faxes, multiple lines, modems, etc.) This I have done with my third graders. I've never had taken it to the next step where students would figure out just how many there are altogether, excluding ones like 555, 800, and 888 because they aren't for AREAs, and ones like 011 and all of the country codes.
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