Several friends went out for a bike trip on a mountain road. On the way up, we all carry a constant speed of 20Km / h. However, when it is time to go down, we do it at 30Km / h.
What is the average speed of the total travel?
Although intuitively it seems that the average speed should be 25Km / h it is actually 24 km / h.
We can see it with a concrete example. Suppose the mountain road is 60 km long. Circulating at 20Km / h it will take 3 hours to go up and only 2 to go down so that we travel 120Km in 5 hours, that is 120Km / 5h = 24Km / h.