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    Question

    Sum of first 20 odd natural numbers is (10m + 200),

    whereas sum of first 24 even natural numbers is (10n + 200). 'p' is the number of factors of (mn - 336). Ratio of milk and water in a mixture is (n + p):(m - p) respectively. When (n - p) litres of water is added, the ratio of milk to water becomes (m + n):3p. Find the final quantity of mixture.
    A 150 lit Correct Answer Incorrect Answer
    B 120 lit Correct Answer Incorrect Answer
    C 144 lit Correct Answer Incorrect Answer
    D 192 lit Correct Answer Incorrect Answer
    E 180 lit Correct Answer Incorrect Answer

    Solution

    Sum of first 'a' odd natural numbers = a²
    So, 10m + 200 = 20²
    Or, 10m + 200 = 400 → 10m = 200 → m = 20
    Sum of first 'b' even natural numbers = b × (b + 1)
    So, 10n + 200 = 24 × 25
    Or, 10n + 200 = 600 → 10n = 400 → n = 40
    Now, mn - 336 = 20 × 40 - 336 = 800 - 336 = 464
    464 = 2⁴ × 29
    Number of factors = (4 + 1) × (1 + 1) = 10
    So, p = 10
    Ratio of initial milk to water = (n + p):(m - p) = (40 + 10):(20 - 10) = 50:10 = 5:1
    Ratio of final milk to water = (m + n):(3p) = (20 + 40):(30) = 60:30 = 2:1
    Let initial milk and water be 5z and z litres.
    n - p = 40 - 10 = 30
    So, 5z:(z + 30) = 2:1 → 5z = 2z + 60 → 3z = 60 → z = 20

    Required mixture = 5z + z + 30 = 6z + 30 = 120 + 30 = 150 litres

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