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 one:  A) 10  B) 20  C) 21  D) 25  E) 24
Explanation:   Suppose the can initially contains 7x and 5x of mixtures A and B respectively.   Quantity of A in mixture left =(7x-7/12 x9)litres =(7x -21/4)litres .   Quantity of B in mixture left =(5x-5/12 x9)litres =(5x -15/4)litres .   (7x -21/4) / (5x -15/4) +9 = 7/9   28x - 21 /20x + 21 = 7/9   252x - 189 = 140x + 147   112x = 336   x = 3.   So, the can contained 21 litres of A.   ANSWER IS 21
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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?