Ratio and Proportion

Concept:Removing a part of the mixture removes components in the original ratio, then adding pure milk increases only the milk quantity.Explanation:The initial mixture of 20 litres has milk and water in ratio $5:3$.Total parts = $5+3=8$.Initial milk = $\frac{5}{8} \times 20 = 12.5$ L.Initial water = $\frac{3}{8} \times 20 = 7.5$ L.When 4 litres of mixture is removed, it also contains milk and water in the same ratio $5:3$.So, milk removed = $\frac{5}{8} \times 4 = 2.5$ L.Water removed = $\frac{3}{8} \times 4 = 1.5$ L.After removal: milk left = $12.5 - 2.5 = 10$ L, water left = $7.5 - 1.5 = 6$ L.Then 4 litres of pure milk is added.New milk = $10 + 4 = 14$ L, water remains $6$ L.New ratio milk:water = $14:6 = 7:3$.Answer:$7:3$