Fill the 5 and pour it into the 3, which leaves 2 in the 5. Empty the 3 and pour the 2 into it. Fill the 5 again and top up the 3, which takes 1 litre. That leaves exactly 4 in the 5-litre jug.
You have a 3-litre jug, a 5-litre jug, neither with any markings, and a tap. How do you measure exactly 4 litres?
Give me a hintThe hint
First work out how to get 2 litres.
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Make 2 litres in the small jug, then top it up from a full big one
Source: Traditional, medieval; famous from Die Hard with a Vengeance, 1995