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      Question

      A bag contains 12 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 36 Correct Answer Incorrect Answer
      D 50 Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      Number of white balls = 12
      Let x be the number of black balls.
      Total number of balls in the bag = 12 + x
      Also, the probability of drawing a black ball from the bag is thrice that of drawing a white ball.
      ⇒ x/(12 + x) = 3[12/(12 + x)]
      ⇒ x = 3 × 12 = 36
      Hence, the number of black balls in the bag = 36

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