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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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