>Here's an trick question: I make a round trip in an automobile between to points (let's call them "A" and "B"). I travel at an average speed of 40mph when going from point A to point B. On the return trip from B to A, I travel at an average speed of 60mph. What is the average speed for the entire round trip? (answer = 48mph)

Good one! In my first year of college I was humbled by this puzzle:

Car travels up a 1 mile hill at 30mph. In order to average 60mph for the entire trip, how fast must the car go down the 1 mile hill?

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Cannot achieve 60 mph for whole trip because that would require the trip to be done in 2 minutes. The 2 minutes were already used going up the hill.