A can contains a mixture of two liquids A and B is the ratio 7 : 5. When 9 litres of mixture are drawn off and the can is filled with B, the ratio of A and B becomes 7 : 9. How many litres of liquid A was contained by the can initially?
Answer: Option C
Suppose the can initially contains 7x and 5x of mixtures A and B respectively.
252x - 189 = 140x + 147
112x = 336
x = 3.
So, the can contained 21 litres of A.