The best riddles. Anywhere.
Cut Along The Tracks
posted by Sifat - September 29 2009 02:40:07 AM
Walking home one day, you take a short cut along the train tracks. The tracks cross a narrow bridge over a deep gorge. At the point you are 3/8 of the way across the bridge, you hear the train whistle somewhere behind you. You charge across the bridge, and jump off the track as the train is about to run you down. As it happens, if you had gone the other way, you would have reached safety just before being run over as well.

If you can run ten miles per hour, how fast is the train moving?
Reply by TokyoQ - April 21 2010 06:47:08 AM
40 miles/hour
Let train be y miles behind start of bridge and bridge be 8x long and train travel at v miles/hour.
Times are equivalent running both ways so:
5x/10 = (y + 8x) / v
3x/10 = y/v so y = .3xv, subbing this gives:
.5x = (.3xv + 8x)/v
.5xv = .3xv + 8x
.2xv = 8x
.2v = 8
v = 40

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