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Probability of throwing dice

  1. Jul 15, 2014 #1
    1. The problem statement, all variables and given/known data

    A pair of dice are thrown. If a die show number 6 , what is the probability of (a)another die show number 2? (b) total score for both number greater than 8 ?

    for part (a) , my working is 1/6 x 1/6 = 1/36

    but actual ans is 2/11 for part (b), the ans is 7/11


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 15, 2014 #2

    LCKurtz

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    Write down all the outcome pairs where at least one die shows a 6. How many such pairs do you have? How many of those pairs also include a 2?
     
  4. Jul 15, 2014 #3
    (1,6) , (2,6) , (3,6) , (4,6) , (5,6) , (6,6)

    one contain (2,6)
     
  5. Jul 15, 2014 #4

    LCKurtz

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    What about (6,1),(6,2) etc.?
     
  6. Jul 15, 2014 #5
    hi, i got the solution now. but i just cant understand why i cant use the probability tree to get the ans as the photo below?
     

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  7. Jul 15, 2014 #6

    vela

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    You drew a tree, but you haven't shown getting an answer to the question using it. What answer do you think the tree is leading you to?
     
  8. Jul 15, 2014 #7

    haruspex

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    Is that the exact wording? If so, it's a terrible question. As LCKurtz presumed, it almost certainly intended "if at least one die shows a 6", but that's not what it says.
    It all hinges on the basis for selecting the die showing a six. The wording suggests that the observer picked one die at random and saw that it was a six. That will not give 2/11 as the answer.
     
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