How much water should be added to 1 gallon of pure antifreeze to obtain a solution that is 60 percent antifreeze?

I understand this to be a mixture problem.

The word PURE tells me that 100 percent somehow applies here.

Here is my attempt. Like always, I seek to know if my equation set up is right or not.

Let w = water that should be added

w + 100 percent (1 gallon) = 0.60(w + 1 gallon)