You drive to a town at 30 km/h and return at 60 km/h. What's your average speed for the round trip?
Hint: It isn't 45.
40 km/h.
Use a 60 km distance: the trip takes 2 h there and 1 h back, so 120 km in 3 h. That's 40 km/h. You spend more time at the slower speed, which pulls the average down.
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