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Alternate answer:

Fill the 3-liter jug, then pour it into the 5-liter jug. The five-liter jug now has 2 liters of air in it. Fill the 3-liter jug, then pour it into the 5-liter jug until it's filled. You now have 1 liter of water remaining in the 3-liter jug. Empty the 5-liter jug, the pour the 3-liter jug's contents into it. Fill the 3-liter again, then pour it into the 5-liter.