You have a 3-liter jug and a 5-liter jug and unlimited water. How do you measure exactly 4 liters?
Hint: Fill the big one first.
Fill the 5L, pour into the 3L (2L left). Empty the 3L and pour the 2L in. Fill the 5L again and top up the 3L (that takes 1L). 4L remains in the 5L jug.
Moving water between the two jugs lets you get amounts neither jug holds on its own. Here 5 − (3 − 2) = 4.
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