A solution contains acid and water in the ratio of 4 : 5. If 20% of the solution is replaced by water, then what will be the ratio of acid and water in the new solution?

Initially, the solution has acid and water in 4:5 ratio, so 40 units acid and 50 units water (total 90 units). Removing 20% means removing 18 units of solution, which contains 8 acid and 10 water (maintaining the 4:5 ratio). After removal: 32 acid, 40 water remain. Adding 18 water gives 32 acid and 58 water. The new ratio is 32:58 = 16:29. The critical step is that the removed portion maintains the original ratio.