Eight litres from a 5 and a 3

Question

You have a 5-litre jug and a 3-litre jug, both unmarked, and a tap. You need to deliver exactly 8 litres of water to a trough a few steps away. Everyone knows the famous version of this puzzle where the target is 4 litres, and it takes several fills and pours. What is the fewest number of moves needed to deliver exactly 8 litres?

Accepted Answer

Two moves: fill the 5 and fill the 3. Carry both to the trough. Five plus three is eight. Why it is worth asking. The puzzle is a trap for anyone who recognises the shape of a problem and reaches for the remembered procedure without reading the numbers. The classic 4-litre version requires clever pouring because 4 is neither a jug size nor a sum of the two jugs. Here the target is simply the total capacity, so the whole "pour back and forth" machinery is unnecessary. The general check. Before solving any measuring puzzle, ask three quick questions. Is the target a jug size? Then one fill. Is …

Eight litres from a 5 and a 3

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