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 ?	Options.
A) 90	B) 2,700	C) 4,500	D) 2,550	E) 324,000

Rationale: "together the two machines make 30 + 55 = 85 copies per minute . so , in half an hour they will make 85 * 30 = 2,550 copies . answer : d ."
Answer: D