A bottle contains 20 litres of liquid A. 4 litres of liquid A is taken out of it d replaced by same quantity of liquid B. Again 4 litre of the mixture is taken out and replaced by same quantity of liquid B. What is the ratio of quantity of liquid A to that of liquid B in the final mixture?

Successive replacement case – If y units of some other liquid is replaced in x units of original liquid n times then the ratio of the quantity of original liquid to the total quantity of liquid after n operation = ((x-y)/x)^n. Applying to our case x = 20, y = 4, n = 2. So at the end of the 2nd operation the ratio of the quantity of liquid A to the total quantity of liquid = It means out of the total quantity of 25 units liquid A is 16 units and liquid B is 26 – 16 = 9 units. Hence required ratio is 16 : 9.