An old chestnut goes like this:

You have two unmarked containers. One is known to hold exactly five liters, the other, three liters. How can you measure exactly four liters of water?

Variations of this exist for other combinations of sizes of containers and amounts to measure, but this seems to be the original.

Answer

Trivia: this one is so old it's even been joked about in the movies. In Die Hard: With a Vengeance the stars have to put exactly four gallons of water on a scale to keep a bomb from exploding, and they are given a five-gallon jug and a three-gallon jug. See "Math in the Movies" at http://world.std.com/~reinhold/dir/mathmovies.html

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