The speed of the jet Alyssa flew from Boston to London was 480 mph. On her return flight, the speed of the jet was 600 mph with a tailwind. What was Alyssa's average speed round trip? Round your answer to one decimal place.

The answer is not 540, you do not calculate the arithmetic mean, also the answer is 533.3, but I do not quite understand how to get the answer. If someone could please explain the process of getting to the answer, that would be very helpful. Thanks.