How do you get exactly 2L if you have a 4L and 5L bucket?
Fill the 4 litre bucket and pour it into the 5 litre. Re-fill
the 4 litre bucket and pour again. That gives you 5 litres (and 3
litres left in the smaller bucket). Empty the 5 litre bucket and
put the remaining contents from the 4 litre into it. fill the 4
litre again - and pour it into the 5 litre bucket until it's full.
Since you can only add 2 litres to the existing 3 litres, what you
have left in the smaller bucket is... 2 litres !