(a) You have a 3-gallon and a 5-gallon jug that you can fill multiple times from a tap. The problem is to measure exactly 4 gallons of water. How do you do it? (b) You have a 9-gallon and a 12-gallon jug that you can fill multiple times from a tap. The problem is to measure exactly 4 gallons of water. Prove that you cannot do it.