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?  Pick one:
A) 10 minutes  B) 20 minutes  C) 25 minutes  D) 30 minutes  E) 35 minutes
Total work done by both machines in a minute = 20 + 10= 
<gadget id="calculator">20 + 10</gadget>
<output>30</output>
30 copies   Total number of copies required= 900   Time = 900/30= 
<gadget id="calculator">900/30</gadget>
<output>30</output>
30 mins   Answer 30 minutes
<result>D</result>
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?