Task: Question Answering
Input (English): Problem: two pipes can fill a tank in 10 minutes and 15 minutes . an outlet pipe can empty the tank in 30 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 ) 7.0 , b ) 7.5 , c ) 8.0 , d ) 8.5 , e ) 9.0
Output (English): b