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Math Help - How Many Phone Numbers?

  1. #1
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    How Many Phone Numbers?

    How can I solve this problem without using the fundamental counting principle?

    The 7-digit phone numbers in a city all have 555 as the first three digits. How many phone numbers are possible?

    I was told this is a permutation NOT a combination word problem.

    In that case, can I apply nPn or nPr. If not, why not?



    WHY WAS THIS QUESTION MOVED FROM BASIC STATISTICS AND PROBABILITY?
    Last edited by sharkman; September 10th 2009 at 01:53 PM.
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  2. #2
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    i think its pretty simple.... you can omit 555 from the problem, as it is redundant and at the start of each number, so you are left with four possible digits. surely you can start at the number 0000, and then 0001, and then 0002 etc... to reach the eventual 9999.

    therefore you can realise that there are only 10000 possibilities. from 0000 to 9999 inclusive.
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