Task: Question Answering
Input (English): Problem: two pipes a and b can fill a cistern in 12 and 15 minutes respectively . both are opened together but after 4 minutes a is turned off . after how much more time will the cistern be filled ? 
Options: a ) 6 , b ) 8 1 / 1 , c ) 8 1 / 4 , d ) 8 1 / 2 , e ) 8 3 / 7
Output (English): a