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Permutations of the letters a, b, c, d, e, f, g

  1. Apr 20, 2004 #1
    How many permutations of the letters a, b, c, d, e, f, g have either two or three letters between a and b?

    My guess for this is that if a and b have to have two letters between them, then there are 5! ways to arrange the rest of the letters, right? Same deal if a and b have to have three letters between them. So:

    5! + 5!

    Would be my answer. Am I anywhere close with this one? Thank you.
     
  2. jcsd
  3. Apr 20, 2004 #2

    mathman

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    Your answer is correct for each specific (a,b) position pair. Your have to multiply by all possible position pairs for a and b, e.g. (1,4), (4,1), (2,5) etc. for two spacing, and similarly for three spacing.
     
  4. Apr 20, 2004 #3
    Thank you.

    I came up with this new answer:

    (5! * 4) + (5! * 3) = 840

    Am I close here?
     
  5. Apr 21, 2004 #4

    mathman

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    You're missing a factor of 2, since a at 1 and b at 4 is different from b at 1 and a at 4, etc., unless you are assuming a is always before b. In that case you are right.
     
    Last edited: Apr 21, 2004
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