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?

1. A Option A: 10
2. B Option B: 20
3. C Option C: 21
4. D Option D: 25

How we write and check questions