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      Question

      A box has โ€˜xโ€™ blue balls and 9 yellow balls. If the

      probability of selecting a blue ball is 3/7, then find the probability of drawing two balls of different colours.
      A 11/21 Correct Answer Incorrect Answer
      B 32/44 Correct Answer Incorrect Answer
      C 21/40 Correct Answer Incorrect Answer
      D 11/12 Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ, Total balls = x + 9 So, x/(x + 9) = 3/7 Or, 7x = 3x + 27 So, x = 27/4 = 6.75 (not possible as balls are integers, so x = 7 approx) Thus, x = 7 Desired Probability = (7C1 ร— 9C1)/(16C2) = (2 ร— 7 ร— 9)/(16 ร— 15) = 21/40

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