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? Choose the correct option: A) 18 B) 19 C) 21 D) 26 E) 28
Suppose the can initially contains 7x and 5x of mixtures A and B respectively.   Quantity of A in mixture left = 7x -	7	x 9 litres = 7x -	21 litres.   12	4   Quantity of B in mixture left = 5x -	5	x 9 litres = 5x -	15 litres.   12	4   7x -	21   4   =	7   5x -	15 + 9   4   9   28x - 21	=	7   20x + 21	9   252x - 189 28  - 21= 
<gadget id="calculator">28  - 21</gadget>
<output>7</output>
7   20x + 21	9   252x - 189= 140x + 147   112x = 336   x = 3.   So, the can contained 21 litres of A.   Option 21
<result>C</result>
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?