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    Question

    A bag contains 10 white and some black balls. If

    the probability of drawing a black ball from the bag is twice that of drawing a white ball, find the number of black balls in the bag.
    A 20 Correct Answer Incorrect Answer
    B 30 Correct Answer Incorrect Answer
    C 25 Correct Answer Incorrect Answer
    D 35 Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Number of white balls = 10
    Let x be the number of black balls.
    Total number of balls in the bag = 10 + x
    Also, the probability of drawing a black ball from the bag is twice that of drawing a white ball.
    ⇒ x/(10 + x) = 2[10/(10 + x)]
    ⇒ x = 2 × 10 = 20
    Hence, the number of black balls in the bag = 20

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