3 & 5 litrer container
easyYou have two unmarked jugs - one holds exactly 3 liters, the other exactly 5 liters - and an unlimited supply of water.
You may fill a jug to the top, empty it completely, or pour from one into the other until either the source is empty or the destination is full. There are no other markings to measure by.
How do you end up with exactly 4 liters?