In a 24 litre milk water solution, where concentration of milk is 250/3%. What amount of water should be added in to the solution so that concentration of milk becomes 50%?

This is impossible as stated. A 24L solution cannot have milk concentration of 250/3% ≈ 83.33% because that would require 20L of milk, but then the concentration would be (20/24)×100 = 83.33%, not 250/3% = 83.33%. The 250/3% notation is garbled. If milk is 20L in 24L solution, to get to 50% concentration by adding water: 20/(24+x) = 0.5, so x = 16L. Answer A matches this.