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?

Here's a brief explanation: 1. Initial quantity of liquid A = 20 litres. 2. After the first operation: 4 litres of A are removed, so 16 litres of A remain. 4 litres of B are added. The mixture now has 16 litres of A and 4 litres of B, totaling 20 litres. The concentration of A is 16/20 = 4/5. 3. After the second operation: 4 litres of the mixture are removed. The quantity of A removed is (4/5) * 4 = 16/5 litres. Quantity of A remaining = 16 - 16/5 = (80 - 16)/5 = 64/5 litres. Then, 4 litres of B are added. This does not change the quantity of A. 4. Final quantities: Quantity of liquid A = 64/5 litres. Total volume remains 20 litres. Quantity of liquid B = Total volume - Quantity of liquid A = 20 - 64/5 = (100 - 64)/5 = 36/5 litres. 5. Ratio of A to B: (64/5) : (36/5) = 64 : 36 Dividing both sides by 4, we get 16 : 9. Therefore, the ratio of liquid A to liquid B in the final mixture is 16:9. The final answer is C) 16:9.