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Probability of holding 3 different posts in a committee

  1. Jul 8, 2014 #1
    1. The problem statement, all variables and given/known data
    A , B and C are from a class of 40 students. Three students are selected to be a monitor, assistant monitor and treasurer.
    a) What's the probability of A to be monitor, B to be assistant monitor , C to be treasurer

    my ans is (1/(40 x 39 x 38))=1/59280

    b. What's the probablity of A , B and C are selected to hold three post?

    can someone help me on part b plaese?

    my working is 40C3=9880

    but the ans is 1/9880


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 8, 2014 #2

    Ray Vickson

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    Presumably you wrote down 40C3=9880 for some reason. What is that reason?
     
  4. Jul 8, 2014 #3
    choose 3 person form 40 person for the 3 posts
     
  5. Jul 8, 2014 #4

    Ray Vickson

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    OK, so now what should you do?

    Personally, I would start from the solution to (a) and think about it a bit more: your solution to (a) is for the ordering (monitor,assistant monitor, treasurer) = (A,B,C). However there are other allowed selections, such as (A,C,B) and several others. They all have probabilities of their own, and their total is what you are after.
     
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