Fill up the 5 litre bottle then pour the water from that into the 3 litre bottle, you'll have 2 litres left in the big bottle. Empty out the small bottle, then pour the 2l into the small bottle. Now you have 2 litres in the 3 l bottle. Now fill up the 5 litre bottle again then use that water to fill up the 3 litre bottle, you will now have 4 litres left in the big bottle.




