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Math Help - tree diagram (i think)

  1. #1
    Super Member
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    Bristol, England
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    tree diagram (i think)

    Hi, this is my question:

    A box contains 10 coloured discs numbered 1 to 10.

    The discs numbered 1 to 5 are red.
    The discs numbered 6 to 9 are blue.
    The discs numbered 10 is green.

    A disc is taken at random from the box and is NOT replaced.
    A second disc is then taken from the box.

    Calculate the probabilitiy that the two discs are the same colour and the numbers on them add to MORE than 8.

    im not really sure what to do on this question, i think i will need a tree diagram. i would really appreciate some help please.

    Thanks
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  2. #2
    Junior Member atalay's Avatar
    Joined
    Apr 2010
    Posts
    27
    4-5, all blue numbers are probable.

    For 4-5= 1/10X1/9= 1/90
    For two balls are blue=  \binom{4}{2} / \binom{10}{2} = 6/90

    So, the answer is 1/90+6/90=7/90
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