If I had 256 fluid ounces of juice how many half gallons would I have?

You would have ##4## half gallons.

##128## US Fluid Ounces are equal to one Gallon.

As ##256=128xx2## fluid ounces are equal to ##2## gallons.

Now each gallon has two half gallon, so ##4## half gallons in ##2## gallons or ##256## fluid ounces.