Task: Question Answering
Input (English): Problem: p can do a work in 15 days and q cando the same work in 20 days . if they can work together for 4 days , what is the fraction of work left ? 
Options: a ) 6 / 15 , b ) 7 / 13 , c ) 8 / 15 , d ) 9 / 17 , e ) 2 / 13
Output (English): c