Task: Question Answering
Input (English): Problem: a mixture contains alcohol and water in the ratio 4 : 3 . if 5 liters of water is added to the mixture , the ratio becomes 4 : 5 . find the quantity of alcohol in the given mixture . 
Options: a ) 10 , b ) 12 , c ) 15 , d ) 18 , e ) 20
Output (English): a