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 ? Choose the correct answer
A) 90	B) 2,400	C) 4,500	D) 5,400	E) 324,000

Rationale: "together the two machines make 25 + 55 = 80 copies per minute . so , in half an hour they will make 80 * 30 = 2,400 copies . answer : b ."
Answer: B