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    Question

    A 30 L solution contains 20% alcohol and the rest water.

    How many litres of water must be added to make the solution 15% alcohol?
    A 10 L Correct Answer Incorrect Answer
    B 18 L Correct Answer Incorrect Answer
    C 21 L Correct Answer Incorrect Answer
    D 17 L Correct Answer Incorrect Answer

    Solution

    ATQ, In 30 L solution, alcohol = 20% of 30 = 6 L Let x L of water be added. Total volume = 30 + x L Alcohol remains 6 L New concentration = 15% = 0.15 So, 6 / (30 + x) = 0.15 6 = 0.15(30 + x) 6 = 4.5 + 0.15x 6 − 4.5 = 0.15x 1.5 = 0.15x x = 1.5 / 0.15 = 10

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