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Counting combinations

  1. Mar 10, 2014 #1
    1. The problem statement, all variables and given/known data

    In how many ways can 4 articles be divided between 2 people when each person must receive at least one article?
    2. Relevant equations



    3. The attempt at a solution

    I tried it like this

    Person 1 could have ( 3,2 or 1 paper(s) )

    Person 2 the same so

    total # ways = 6 + 6 = 12

    But this is wrong.
     
  2. jcsd
  3. Mar 10, 2014 #2

    Simon Bridge

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    The number of articles each person has is not independent.
    It is not clear that the articles are identical - suspect not.

    Label the articles 1,2,3,4
    (note: an "article" need not be a paper - it can be any object, like "an article of clothing".)

    Label people as A and B (Alice and Bob, say).

    Now list the different ways Alice and Bob can have some stuff.
    Start by giving Alice one article and Bob the rest.
    Then give Alice two articles, then three.
     
  4. Mar 10, 2014 #3
    So like A-1, B-3 = 3 ways
    A-2, B-2 = 4 ways

    A-3,B-1 = 3 ways
    So total # = 10 ways?
     
  5. Mar 10, 2014 #4
    Nevermind I got it thanks.
     
  6. Mar 11, 2014 #5

    Simon Bridge

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    Well done: what did you come up with?
    (JIC someone else gets stuck - then they benefit from your efforts)
     
  7. Mar 11, 2014 #6
    I had the first possibility ( 4C1 x 3C3)
    2nd (4C2 x 2C2)
    3rd (4C1 x 3C3)

    Then I added them up to get the total number of ways of carrying out the selections. I got 14.
     
  8. Mar 11, 2014 #7

    Simon Bridge

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    Cool - what made you suddenly switch to combinations notation?

    The question amounts to asking the number of ways Alice can pick at most 3 articles out of 4 when the order doesn't matter.

    That's (4x3x2)/(3x2)+(4x3)/(2)+4=4+6+4=14.

    There's so few you can just list them.
     
  9. Mar 11, 2014 #8
    Usually when I can't count it out lol I try Combinations, some times it just seems easier with combinations.
     
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