That is 640 ounces.
5
4 gal = 512 fl oz
(6 oz per 100 gal) times (20) = (120 oz per 100 gal), or (1.2 oz per gal) .
Assuming you mean 12.6 fl oz, I guess you're American (using the oz abbreviation for fl oz), thus: 1 gal = 128 fl oz → 5 gal = 5 × 128 fl = 640 fl oz → 640 fl oz ÷ 12.6 fl oz ≈ 50.79
1 US gal is 128 fluid ounces, so 2 gal is 256 fl oz.
That is 256 fl oz.
That is 192 fl oz.
That is 192 fl oz.
That is 64 fl oz.
2
256
3.56 gal x 128 fl-oz/gal = 455.68 fl oz