Task: Question Answering
Input (English): Problem: a can do a piece of work in 4 days . b can do it in 15 days . with the assistance of c they completed the work in 24 days . find in how many days can c alone do it ? 
Options: a ) 0.391 days , b ) 0.491 days , c ) 0.331 days , d ) 1.391 days , e ) 0.301 days
Output (English): a