C. 21
Feedback Suppose the can initially contain 7x and 5x liters of mixtures A and B respectively
Quantity of A in mixture left =(7x - 7/12 x 9) = (7x - 21/4) litres.
Quantity of B in mixture left = (5x - 5/12 x 9) = (5x - 15/4) litres.
(7x - 21/4)/[(5x - 15/4 + 9)] = 7/9
=> 252x -189 = 140x + 147
=> x =3.
So, the can contain 21 liters of A.