First you should all switch to metric-) 320oz for 2.5 gallons is sure to confuse us who grew up with imperial gallons.
I don't think Jorge's math is right. What you know is 1 part is 32oz. 9parts is therefore 32x9. The total being 320. 1 gallon is what 128oz in the US?? therefore 1part would be 12.8 oz the rest would be water.
This is the same thing Tschmid said.
The easy thing would have been to divide 32oz by 2.5.