A cyclist pedals up a mountain road at a steady 10 miles per hour, then turns directly around and descends along the identical route at 30 miles per hour. What was the cyclist's average speed for the full round trip?
15 miles per hour
Average speed over identical distances is the harmonic mean of the two speeds: (2 * 10 * 30) / (10 + 30) = 600 / 40 = 15 mph.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Medium. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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