Task: Question Answering
Input (English): Problem: a cistern is filled by pipe a in 12 hours and the full cistern can be leaked out by an exhaust pipe b in 18 hours . if both the pipes are opened , in what time the cistern is full ? 
Options: a ) 50 hrs , b ) 60 hrs , c ) 36 hrs , d ) 80 hrs , e ) 90 hrs
Output (English): c