Task: Question Answering
Input (English): Problem: a and b can do a piece of work in 10 days , b and c in 5 days , c and a in 15 days . how long will c take to do it ? 
Options: a ) 77 days , b ) 55 days , c ) 12 days , d ) 33 days , e ) 11 days
Output (English): c