You have an unlabeled 3-liter jug and an unlabeled 5-liter jug, along with an unlimited supply of water. How can you measure out exactly 4 liters using only these jugs?
Fill the 5L jug, pour water into the 3L jug until full (leaving 2L in the 5L jug). Empty the 3L jug, pour the 2L into the 3L jug. Fill the 5L jug completely, then pour into the 3L jug until full (1L transfers). Exactly 4L remains in the 5L jug.
Filling 5L and pouring into 3L leaves 2L in the 5L jug. Transferring that 2L to the empty 3L jug leaves 1L of space in the 3L jug. Filling 5L and topping off the 3L jug removes 1L from the 5L jug, leaving exactly 4L.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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