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    Question

    A jar contains 'z' white balls and 9 black balls. If the

    probability of getting a white ball is 4/7, then find the probability of getting different coloured balls when two balls are drawn randomly from the jar.
    A (12/35 Correct Answer Incorrect Answer
    B (3/7 Correct Answer Incorrect Answer
    C (18/35 Correct Answer Incorrect Answer
    D (3/5 Correct Answer Incorrect Answer
    E (24/35 Correct Answer Incorrect Answer

    Solution

    Total number of balls in the jar = z + 9
    So, z/(z + 9) = 4/7
    Or, 7z = 4(z + 9)
    Or, 7z = 4z + 36
    So, z = 12
    Total balls = 12 + 9 = 21
    Desired Probability = (12C1 × 9C1)/21C2 = (12 × 9)/(21 × 20/2) = 108/210 = 18/35

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