Student: One copy machine can make 20 copies a minute, and a second copy machine makes 10 copies a minute. If the two copiers work together, how long would it take them to make 900 copies?
Options:
(A) 10 minutes
(B) 20 minutes
(C) 25 minutes
(D) 30 minutes
(E) 35 minutes.
Another student: Let's say, hmmm...
Total work done by both machines in a minute = 20 + 10 = 30 copies
Total number of copies required = 900
Time = 900/30 = 30 mins Final answer: (D).