When a quantity of mixture is withdrawn and replaced by a constituent, the remaining quantity of the original constituent can be calculated. After the first operation, the quantity of milk remaining is 24 * (1 - 4/24) = 20 litres. After the second operation, the quantity of milk remaining is 20 * (1 - 4/24) = 20 * (5/6) = 100/6 litres. The total volume of the mixture remains 24 litres. Thus, the quantity of water is 24 - 100/6 = (144 - 100) / 6 = 44/6 litres. The ratio of milk to water is (100/6) : (44/6), which simplifies to 100 : 44, and further to 25 : 11.