A mixture contains only 10 litres of milk and the rest is water. A new mixture is formed by replacing mixture with pure milk whose concentration is 40%. How many litres of mixtures was replaced if initially there were 40 litres of water in the mixture?
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10 litre
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12 litre
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9 litre
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11.5 litre
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12.5 litre
E
Correct answer
Explanation
Initial: 10L milk + 40L water = 50L mixture (20% milk). Replace x liters with pure milk to get 40% milk. Remaining milk = 10 - 0.2x, new milk added = x. Final milk = 10 - 0.2x + x = 10 + 0.8x. Final volume = 50L. For 40% concentration: (10 + 0.8x)/50 = 0.4, giving 10 + 0.8x = 20, so x = 12.5L.