First, multiply the number of feet it takes for one balloon by the number of balloons filled daily.
0.5 * 80 = 40
So, 40 cubic feet is 20% of one tank. Now, unless you remember how to find what 40 is 20% of, simply play around with some numbers on your calculator until you find that 200 * 0.20 = 40. I would love to explain how to find that part with an actual formula, but I don't remember how. :/ Hope this helped.
