Kordemsky's Two Passing Freight Trains
Two freight trains, each a sixth of a mile long, travel toward each other at 60 mph apiece. From the instant their locomotives draw level to the instant their cabooses do, how many seconds pass?
Reveal the answer
Ten seconds. Together the trains must cover their combined length (1/3 mile) at their combined closing speed (120 mph): (1/3 mile) ÷ (120 mph) = 1/360 hour = 10 seconds. It's one of the relative-motion brainteasers in Boris Kordemsky's 'The Moscow Puzzles' (1956), the best-selling puzzle book in Soviet history, later edited for English readers by Martin Gardner.
— Boris A. Kordemsky, The Moscow Puzzles: 359 Mathematical Recreations — First published in Russian, 1956 (as Matematicheskaya Smekalka); English edition ed. Martin Gardner, 1972
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