A merchant has a 4-gallon jug and a 9-gallon jug, neither with any measurement markings. How many exact operations of filling or pouring are required to measure out precisely 6 gallons of water using only these two jugs?
6 operations
Fill 9-gal (1), pour from 9 to 4 until 4 is full (2), empty 4-gal (3), pour remaining 5 gal from 9 to 4 (4), fill 9-gal (5), pour from 9 to 4 until 4 is full (6). The 9-gal jug now holds exactly 6 gallons.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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