You are thinking of purchasing an aquarium. You contact the owner of an aquarium store. You need to know how many fish to purchase for an aquarium, but first you must determine how big the aquarium will be. The owner of the aquarium store tells you his rule of thumb is to purchase "a total length of fish of 3 inches for each 2 gallons of water in the aquarium.'' How many gallons of water would you need if you had a 4-inch fish and a 2-inch fish?