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Homework Help: A question about dice probability calculation

  1. Nov 28, 2007 #1
    A question about probability calculation
    What is the probability that the sum from two dices are under 9, i don’t know a fast way to calculate that :(, instead i calculated it manually by listing all possible event.
    Divided by the total number of outcomes 6*6=36

    And something else that I can not understand,
    p=probability that this happen

    Case 1
    dice (1,1), dice (1,1), <-> is one event <-> 2p
    case 2
    dice(1,2) and dice(2,1) <-> is two events <->2p

    How come case one gets 1 event and 2p when case 2 gets two events and 2p ...

    Kindly TNS ...
  2. jcsd
  3. Nov 28, 2007 #2


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    Homework Helper

    If you get more than 9(assuming that you meant sum>9 and not equal to 9) you can get (6,3) or (6,4) or (6,5) or (6,6) or (5,5) or (5,4) <---- these gives you the possibilities to get a sum[itex]\geq[/itex]9. NOTE: this follows the order, (dice1,dice2)

    so to get (6,3)=[itex]\frac{1}{6}*\frac{1}{3}*2!=\frac{1}{9}[/itex]

    since all the possibilities will be similar to that it will simply be [itex]\frac{1}{9}*6[/itex] which is [itex]\frac{2}{3}[/itex] but this is to find P(sum[itex]\geq[/itex]9) so then to find what you want is simply 1-P(sum[itex]\geq[/itex]9)
  4. Nov 28, 2007 #3


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    Science Advisor

    If you were calculating the probability of getting a 2 on a pair of dice, there is only one way that can happen. Die "A" is a 1 and die "B" is a 1. Probability of rolling a 2 is 1/36.
    If you were calculating the probability of getting a 3 on a pair of dice, there are two ways that can happen. Die "A" is a 2 and die "B" is a 1 or die "A" is a 1 and die "B" is a 2. Probability of rolling a 3 is 2/36= 1/18.
  5. Nov 28, 2007 #4
    Thank you very much for your replies, i think it is clear now.

    Kindly TNS
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