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You have a 3 gallon jug and a 5 gallon jug. You need to measure out exactly 7 gallons of water. How can you do it?
What is Fill the 5 gallon jug, pour it into the 3 gallon jug until the 3 gallon is full, leaving 2 gallons in the 5 gallon jug. Now pour the 3 gallon jug out. Pour the remaining 2 gallons from the 5 gallon into the empty 3 gallon jug. Now fill the 5 gallon from the faucet. You now have exactly 7 gallons.