If 1 gallon is equal to 128 fluid ounces, and you already have 64 fluid ounces, you would need:
128 fl oz (1 gallon) – 64 fl oz (what you have) = 64 fl oz
So, you would need an additional 64 fluid ounces to equal 1 gallon.
If 1 gallon is equal to 128 fluid ounces, and you already have 64 fluid ounces, you would need:
128 fl oz (1 gallon) – 64 fl oz (what you have) = 64 fl oz
So, you would need an additional 64 fluid ounces to equal 1 gallon.