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      Question

      There are black and white marbles in a jar in the ratio

      4:5. After 20 marbles of each type are added, the ratio becomes 6:7. Find the original number of white marbles.
      A 50 Correct Answer Incorrect Answer
      B 30 Correct Answer Incorrect Answer
      C 20 Correct Answer Incorrect Answer
      D 10 Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let the original number of black and white marbles be 4x and 5x.

      ATQ:

      (4x + 20)/(5x + 20) = 6/7

      β‡’ 7(4x + 20) = 6(5x + 20)

      β‡’ 28x + 140 = 30x + 120

      β‡’ 2x = 20

      β‡’ x = 10

      Original white marbles = 5 Γ— 10 = 50

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