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Probability Dice Question

  1. Oct 24, 2009 #1
    1. The problem statement, all variables and given/known data

    A die is rolled 13 times. What is the probability of at least two 6's appearing? (Round your answer to four decimal places.)

    2. The attempt at a solution
    I know that the total number of outcomes is 6^13. I did (13nCr2)x(1x1x6x6x6x6x6x6x6x6x6x6x6)/6^13, but the answer isn't right. What am I doing wrong?
  2. jcsd
  3. Oct 24, 2009 #2
    Perhaps you should try explaining the theory behind your solution.
    I think working on the complement would be much easier. That is, finding the number of times a 6 would appear 0 times and the number of times a six would appear 1 time, and then take the complement.
  4. Oct 24, 2009 #3


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    Homework Helper

    What is the difference between "at least two 6s appear" and "exactly two sixes appear?" which did you calculate?
    VeeEight's suggestion is spot on.
  5. Oct 24, 2009 #4

    I see. I calculated the number of times exactly two sixes would appear. Not quite sure how to do the compliment though.
  6. Oct 25, 2009 #5
    You can do separate cases (exactly two sixes, exactly three sixes, etc) and add them all up. It's just that taking the complement is less work
    (a good exercise might be to do both to make sure they are the same)
  7. Oct 25, 2009 #6


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    Science Advisor

    The complement of "at least two sixes" is "0 or 1 sixes" and is easier to calculate sinced it involves only two cases rather than 5.
  8. Oct 25, 2009 #7
    Thanks! Got it.
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