Last week I travelled from London to Leeds, which is a distance of 174 miles. I started at 9.15am and completed the journey with an average speed of 40 miles per hour. On the way back, in the evening, I travelled exactly the same route, starting at 5.15pm. The traffic was light and I completed the journey with an average speed of 60 miles per hour. What was the overall average speed for round trip? The answer is not 50 mph.