Math & Logic · Ages 12–15
Riddle #149
You have an unmarked 3-liter jug and an unmarked 5-liter jug. How can you measure out exactly 4 liters of water?
You solved this one before
Open-ended riddle: think it through, then reveal the answer.
Answer: Fill the 5L jug, pour into the 3L jug until it's full (2L stays in the 5L jug). Empty the 3L jug, then pour that 2L into it. Fill the 5L jug again, then top off the 3L jug (which needs 1 more liter) from it — that leaves exactly 4L in the 5L jug.