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Rolling dice until sum is 14

  1. Mar 17, 2009 #1
    1. The problem statement, all variables and given/known data

    Roll 7 dice. What is the probability that the sum of the numbers is 14?


    2. Relevant equations

    Possibly a probability generating function because the problem comes from the chapter that deals with it but I can't think of which one (if any actually apply).


    3. The attempt at a solution

    Here is the solution: 6-7[(13 choose 7)-49]

    I really have no idea why. The 67 in the denominator I understand because that is the total number of different ways you can roll 7 dice. Where does the rest come from?
     
  2. jcsd
  3. Mar 17, 2009 #2
    By the definition of probability, the numerator should be the number of different ways you can roll seven dice and get a sum of 14. So how would you count them to get this answer?
     
  4. Mar 17, 2009 #3

    Mark44

    Staff: Mentor

    You might start with a simpler problem to get your head around the harder problem. For example, what's the probability of rolling three dice and getting a total of 6?

    The only combinations that produce a total of 6 are (2, 2, 2) and (1, 2, 3). There is only one way to get (2, 2, 2), namely that all three dice have to be 3s, but there are 6 ways to get (1, 2, 3). I've written this as an ordered triple, but I really don't mean it that way--only that one die is a one, another is a two, and the last is a three.

    So for this easier problem, what's the probability that three dice will show a total of 6? Can you extend this idea to your problem?



    Can you extend this counting strategy to
     
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