if 12 ounces treats 15 gallons, 15 divided by 5 is 3, so just divide 12 by 3, and 4 is your answer.
32 ounces treats 4,000 gallons.
8.5333333333333 ounces
If 1 oz treats 2.5 gallons then all you need to do is divide 1 by 2.5 to find out how much is required for 1 gallon. The answer would be 2/5ths of an ounce, which is 0.4 oz.
For swimming pools one big (3 inch) tablet treats about 8000 gallons
20 gallons = 2,560 ounces.
13.2 gallons is 1,689.6 ounces.
8320 ounces is 65 gallons.
169 ounces is 1.32 US gallons.
63 US gallons is 8,064 ounces.
5.22 fluid ounces is 0.04 gallons.
Eleven US gallons is 1,408 ounces.
24 US gallons is 3,072 ounces.