Task: Question Answering
Input (English): Problem: two pipes a and b can fill a cistern in 20 and 30 minutes respectively , and a third pipe c can empty it in 60 minutes . how long will it take to fill the cistern if all the three are opened at the same time ? 
Options: a ) 15 min , b ) 15 1 / 7 min , c ) 17 1 / 7 min , d ) 7 1 / 7 min , e ) 7 2 / 7 min
Output (English): a