Task: Question Answering
Input (English): Problem: two pipes can fill a tank in 18 minutes and 60 minutes . an outlet pipe can empty the tank in 45 minutes . if all the pipes are opened when the tank is empty , then how many minutes will it take to fill the tank ? 
Options: a ) 1 / 15 , b ) 1 / 16 , c ) 1 / 20 , d ) 1 / 10 , e ) 1 / 12
Output (English): c