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Combinatoric question

  1. Feb 18, 2012 #1
    in a three floor building on each store we have 3 appartments.
    so in total we have 9 appartments.

    every appartment has one person.

    3 people enter the elevator(from the entrace of th building),each one goes to the floor he lives in.
    what are the odds that the elevator will stop at each floor
    what are the odds that the elevator will stop at two floors
    what are the odds that in the elavator would be 2 tenants from the second floor

    regarding A:
    we have 3 places to put 3 people so
    for the first from we have 3 options the secong one 2 options and on the last floor we have only one person.

    so its


    and i dont know why all the oprions(the expresion in the denominator) is
    9 over 3
  2. jcsd
  3. Feb 18, 2012 #2
    denominator: 3 person going to 9 places, i.e., choose 3 places from 9 for 3 person, does order matter?
    numerator: 1st person has 9 places to go, 2nd has ? 3rd has ?
  4. Feb 21, 2012 #3
    9*8*7 / (9 over 3)

    regarding part B

    we choose 2 floors from 3 (3 over 2)
    we have three person the first has 6 places the second has 5 third is 4

    (3 over 2)*6*5*4 /(9 over 3)

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