A 20-liter mixture contains milk and water in the ratio 3:1. How much water must be added to make the ratio of milk to water 2:1?

✓ Correct Answer: Option B

Initially, milk = (3/4)*20 = 15L and water = (1/4)*20 = 5L. Let x be the water added. New ratio: 15 / (5+x) = 2/1. This gives 15 = 10 + 2x, so 2x = 5, and x = 2.5 liters.