A drum contains 40 litres of milk. One-fourth of the milk is taken out and replaced with water. Then, half of the mixture is taken out and replaced with water. What is the respective quantity of milk and water left in the drum?

Initial: 40 liters milk, 0 water. After removing 1/4 and adding water: milk = 40 - 10 = 30 liters, water = 10 liters. After removing half the mixture and adding water: half of 40 = 20 removed, which contains proportionally 15 milk and 5 water. Remaining: 15 milk, 5 water. Adding 20 water: milk = 15 liters, water = 5 + 20 = 25 liters. Final: 15 milk, 25 water.