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    Question

    A bag contains 15 white and some black balls. If

    the probability of drawing a black ball from the bag is thrice that of drawing a white ball, find the number of black balls in the bag.
    A 20 Correct Answer Incorrect Answer
    B 45 Correct Answer Incorrect Answer
    C 10 Correct Answer Incorrect Answer
    D 50 Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Number of white balls = 15
    Let x be the number of black balls.
    Total number of balls in the bag = 15 + x
    Also, the probability of drawing a black ball from the bag is thrice that of drawing a white ball.
    ⇒ x/(15 + x) = 3[15/(15 + x)]
    ⇒ x = 3 × 15 = 45
    Hence, the number of black balls in the bag = 45

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