If you have two jugs, one that is 5 gallons and the other 3 gallons, how do u get 4 gallons?

Fill the 5 gallon twice with the 3 gallon, you should have 1 gallon left over in the 3 gallon after the 2nd fill. Empty the 5 gallon and then pour the 1 gallon into it. Then refill and pour the 3 gallon into the 5 gallon.