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Homework Help: Counting and Probability

  1. Apr 1, 2008 #1
    A seven-digit phone number in the United States consists of a three-digit exchange followed by a four-digit number. How many exchanges are needed to serve a city of 80,000 people?

    Combination, Permutation, and arrangement with repetition equations are used in this section.

    Part 1 of this question I got right: 8,000,000 people can have phone numbers in one area code if the first digit of the 7 numbers can't be 0 or 1. Part 2 listed above I'm absolutely stumped on though.
  2. jcsd
  3. Apr 1, 2008 #2
    10,000 numbers per exchange?
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