Pour 5 litres into the 3 litre bottle, this leaves 2 litres in the the 5 litre bottle. Empty the 3 litre bottle and pour the 2 litres from the 5 litre bottle into the 3 litre bottle. Then fill up the 5 litre bottle and fill up the 3 litre bottle which has space for 1 litre, this then leaves 4 litres in the 5 litre bottle.
