Task: Question Answering
Input (English): Problem: two pipes a and b can separately fill a cistern in 60 minutes and 80 minutes respectively . there is a third pipe in the bottom of the cistern to empty it . if all the three pipes are simultaneously opened , then the cistern is full in 40 minutes . in how much time , the third pipe alone can empty the cistern ? 
Options: a ) 90 min , b ) 100 min , c ) 240 min , d ) 120 min , e ) 130 min
Output (English): c