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      Question

      There are is chest containing only diamond and emeralds

      in the ratio 2:5. When 2 of them are picked at random form the chest, the probability that both are diamonds is 1/14. What is the difference between the number of diamonds and emeralds?
      A 12 Correct Answer Incorrect Answer
      B 8 Correct Answer Incorrect Answer
      C 15 Correct Answer Incorrect Answer
      D 7 Correct Answer Incorrect Answer
      E 9 Correct Answer Incorrect Answer

      Solution

      Let the number of diamonds and emeralds in the chest be 2k and 5k respectively.

      P(2 diamonds) = 1/14

      ( 2k C2 )/( 7k C2 ) = 1/14

      2k(2k - 1)/7k(7k - 1) = 1/14

      7k - 1 = 4(2k - 1)

      7k - 1 = 8k - 4

      k = 3

      Required difference = 5k - 2k = 9

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