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  1. #1
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    confusion

    In a bottle of 3 red, 4 green and 5 blue marbles, if 2 marbles are taken out, what is the probability that two marbles are of red color


    my answer is 1/4 i just did like number of red balls by total number balls .


    i have another confusion like 3c2/12c2 which is 1/22...which is correct
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  2. #2
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    Quote Originally Posted by bluffmaster.roy.007 View Post
    In a bottle of 3 red, 4 green and 5 blue marbles, if 2 marbles are taken out, what is the probability that two marbles are of red color


    my answer is 1/4 i just did like number of red balls by total number balls .


    i have another confusion like 3c2/12c2 which is 1/22...which is correct
    I assume the balls are taken out without replacement. In which case a tree diagram easily eveals the answer: \frac{3}{12} \cdot \frac{2}{11} which can be simplified.
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  3. #3
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    Quote Originally Posted by bluffmaster.roy.007 View Post
    In a bottle of 3 red, 4 green and 5 blue marbles, if 2 marbles are taken out, what is the probability that two marbles are of red color


    my answer is 1/4 i just did like number of red balls by total number balls .


    i have another confusion like 3c2/12c2 which is 1/22...which is correct
    There are a total of 3+4+5 = 12 marbles in the bag, 3 of these are red.

    Taking out one marble means there is a \frac{3}{12} = \frac{1}{4} chance that it is red. Also there are now only 11 marbles in the bag

    The chance that this is also red is \frac{2}{11} because there are only 2 red marbles as the previous one was red.

    Since these are an "and" example you multiply these together to give the chance of two red ones: P(RR) = \frac{1}{4} \times \frac{2}{11}
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