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Math Help - Combinatorics

  1. #1
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    Combinatorics

    • Eight speakers
    • Richard wnats to sit beside Hand
    • Thomas and Lily cannot sit together


    How many ways can Lisa make the seating plan?

    Many thanks,

    Needs Help
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  2. #2
    MHF Contributor undefined's Avatar
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    Quote Originally Posted by NeedsHelpp View Post
    • Eight speakers
    • Richard wnats to sit beside Hand
    • Thomas and Lily cannot sit together


    How many ways can Lisa make the seating plan?

    Many thanks,

    Needs Help
    Are they to be arranged in a circle or in a line? (In a circle, ABCDEFGH is the same as BCDEFGHA)
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  3. #3
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    At a table, so circle. I guess?
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  4. #4
    MHF Contributor undefined's Avatar
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    Quote Originally Posted by NeedsHelpp View Post
    At a table, so circle. I guess?
    I would start by fixing the locations of Richard and Hand (did you mean Hank?), keeping in mind that their positions can be switched, so multiply by 2. Now we are dealing with a linear (non-circular) permutation of the remaining 6 speakers, with the sole restriction: Thomas and Lily cannot sit together. I'd then take total permutations, 6!, and subtract those that place Thomas and Lily together. There are 5 ways to place Thomas and Lily together irrespective of order, and 10 considering order. For each of those 10, there are 4! ways to permute the others. So we have

    Answer = 2 * (6! - (10 * 4!)) = 960 ways
    Last edited by undefined; June 18th 2010 at 08:26 PM. Reason: clarification
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