Answer to old chestnut: burning two ropes:

Initially, light both ends of one rope and one end of the other. After 30 minutes, you will have burned 30 minutes' worth of rope from each end of the first rope, or thus one hour worth of rope -- the entire rope. So when the flames from the first rope meet and extinguish themselves, light the other end of the second rope. 30 minutes worth of rope will remain, but this will burn in 15 minutes when burning it from both ends.

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