A copy machine, working at a constant rate, makes 30 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 ? Choices:  A) 90  B) 2,700  C) 4,500  D) 2,550  E) 324,000
Together the two machines make 30+55= 
<gadget id="calculator">30+55</gadget>
<output>85</output>
85 copies per minute. So, in half an hour they will make 85*30=2,550 copies.   Answer: 2,550.
<result>D</result>
A copy machine, working at a constant rate, makes 30 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 ?