Task: Question Answering
Input (English): Problem: two pipes a and b can fill a cistern in 9 and 18 minutes respectively , and a third pipe c can empty it in 24 minutes . how long will it take to fill the cistern if all the three are opened at the same time ? 
Options: a ) 6 min , b ) 7 min , c ) 8 min , d ) 10 min , e ) 17 min
Output (English): c