Ask people the following problem, and almost all of them will get it wrong: “If a car goes up a hill at 20 m.p.h. and comes down the same hill at 30 m.p.h., what is the average rate of speed for the whole trip?”
The “obvious” answer is 25 m.p.h., but that would be wrong. Take a quick look at a slideshow of some of the projects my students have done to see some nice explanations of confounding problems like these.
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Simply stated, since the car spends more time traveling at the slower speed, this has the effect of reducing the average rate of speed below the average of 20 and 30. The surprising answer to this poser is 24 m.p.h.
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