Task: Question Answering
Input (English): Problem: a copy machine , working at a constant rate , makes 25 copies per minute . a second copy machine , working at a constant rate , makes 55 copies per minute . working together at their respective rates , how many copies do the two machines make in half an hour ? 
Options: a ) 90 , b ) 2,400 , c ) 4,500 , d ) 5,400 , e ) 324,000
Output (English): b