Measure exactly 6 litres with a 9 and a 4 jug

Question

You are handed a 9-litre jug and a 4-litre jug, both completely unmarked. There is a tap that can fill either jug to the brim, a drain that can empty either jug, and you may pour one jug into the other until the source runs out or the target is full. You can never judge a level by eye. Using only these moves, leave exactly 6 litres of water in one of the jugs.

Accepted Answer

Aim to finish with 6 litres in the 9-litre jug. Track the pair (9-jug, 4-jug): 1. Fill the 9-jug from the tap. State: (9, 0). 2. Pour into the 4-jug until it is full, 4 litres move. State: (5, 4). 3. Empty the 4-jug down the drain. State: (5, 0). 4. Pour into the 4-jug again, 4 more litres move. State: (1, 4). 5. Empty the 4-jug. State: (1, 0). 6. Pour the 9-jug's 1 litre into the 4-jug. State: (0, 1). 7. Fill the 9-jug from the tap. State: (9, 1). 8. Pour into the 4-jug until full, the 4-jug already holds 1, so only 3 litres move. State: (6, 4). The 9-litre jug now holds exactly 6 litres. Wh…

Measure exactly 6 litres with a 9 and a 4 jug

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