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Answers: you have a five gallon jug and a three gallon jug. You need exactly 4 gallons of water how do you do it?

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Or if you have no third container, but are allowed to return liquid to the main reservoir or discard it, do this:

1) Fill 3-gallon, then pour this into 5-gallon.
2) Refill 3-gallon, then pour as much from the 3-gallon into the 5 gallon as you can. At this point, you have 5-gallons in the large jug and 1 gallon in the small jug.
3) Empty the 5 gallon jug, then pour the contents of the smaller jug (1 gallon) into the large one.
4) Refill the small jug (3-gallon), then pour this into the large jug. The large jug now has 4-gallons of liquid.

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Do you have a third container to put the 4 gallons of water in? If so fill the 5 gallon jug. Use that to fill the 3gallon jug. That will leave 2 gallons in the 5 gallon jug. Pour that into the third container. Repeat and that will give you 4 gallons in the third container.

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fill 3 gallon jug, pour water into 5 gallon jug. fill 3 gallon again, pour water into 5 gallon to fill. you have 5 gallons in the 5 gallon jug and 1 gallon in the 3 gallon jug. empty the 5 gallon jug, pour the gallon from the 3 gallon jug into the large one. you now have 1 gallon in the 5 gallon jug, and an empty 3 gallon jug. fill the 3 gallon jug, pour into the 5 gallon jug. The 5 gallon jug now has 4 gallons.

or:

fill the 5 gallon jug, use it to fill the 3 gallon jug. empty the 3 gallon jug, pour the remaining 2 gallons from the 5 gallon jug into the 3 gallon jug. fill the 5 gallon jug. you now have 2 gallons in the 3 gallon jug and 5 gallons in the 5 gallon jug. pour from the 5 gallon jug into the 3 gallon jug to fill. you now have 4 gallons in the 5 gallon jug

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