Task: Question Answering
Input (English): Problem: a can contains a mixture of liquids a and b is the ratio 7 : 5 . when 12 litres of mixture are drawn off and the can is filled with b , the ratio of a and b becomes 7 : 9 . how many liter of liquid a was contained by the can initially ? 
Options: a ) 25 , b ) 27 , c ) 29 , d ) 28 , e ) 31
Output (English): d