The harmonic mean
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There are many ways to calculate an "average." Imagine following a winding path to the top of a hill at 4 km/h on the way up and 6 km/h on the way back. What is your average speed? The tempting answer is 5 km/h, but that is actually the average of the two speeds, not your average speed!
If the path is 1 km long, the outward journey takes fifteen minutes and the return journey ten minutes, for a total of twenty-five minutes, or of an hour.
Overall, your average speed is 2 / (25/60) = 4.8 km/h.
This particular mean is the harmonic mean:
