Task: Question Answering
Input (English): Problem: two pipes a and b can fill a tank in 18 hours and 24 hours respectively . if both the pipes are opened simultaneously , how much time will be taken to fill the tank ? 
Options: a ) 20 hours , b ) 15 hours , c ) 10 hours , d ) 10 2 / 7 hours , e ) 8 hours
Output (English): d