Problem:

You have a 3-gallon jug and a 5-gallon jug, and you need to measure exactly 4 gallons of water. How do you do it?

Scroll to the bottom of the page for the answer

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Solution:

Fill the 5-gallon jug, pour water into the 3-gallon jug until it is full, leaving 2 gallons in the 5-gallon jug. Empty the 3-gallon jug, pour the 2 gallons from the 5-gallon jug into the 3-gallon jug. Fill the 5-gallon jug again, and fill the 3-gallon jug from the 5-gallon jug (which already has 2 gallons), leaving exactly 4 gallons in the 5-gallon jug.