I'll get back to you with the answer later...
3 & 5
Fill the 5 gallon, and pour it into the 3.
You now have 2 gallons left over in the 5.
Empty the 3, and pour the 2 gallons from the 5 into the 3, and then fill the 5 again.
Pour the 5 gallon into the 3 gallon until the 3 gallon is filled.
There were 2 gallons in there, and you have just added 1 more gallon, filling it at 3.
When you emptied that 1 gallon from the 5 gallon, that leaves you with 4.
