Juan Rivera drove 80 miles into the country at an average rate of 55 mi/h. He drove back on the same highway at an average rate of 45 mi/h. What was his average rate of speed for the entire round trip? (hint : the answer is not 50 mi/h)

Juan Rivera drove 80 miles into the country at an average rate of 55 mph.
He drove back on the same highway at an average rate of 45 mph.
What was his average rate of speed for the entire round trip? (Hint : the answer is not 50 mph)