32 gallons from the 1st brand. 48 gallons from the 2nd brand will do it.
If you will have a unit (volume) of final product, you can write down the equation as:
(Note that x is the 1st brand's contribution (volome) and (1-x) is 2nd brand's contribution)
If you want to have 80 gallons (total final product), you are going to multiply 80 by these numbers. Therefore,
You need to take 32 gallons from Brand 1 and you need to take 48 gallons from Brand 2. Total volume of final product will be 80 gallons and its antifreeze content will be 65%.