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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

    ⇒ 7(4x + 20) = 6(5x + 20)

    ⇒ 28x + 140 = 30x + 120

    ⇒ 2x = 20

    ⇒ x = 10

    Original white marbles = 5 × 10 = 50

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