Originally Posted by mpolli

I get the same answer, but I think a simpler approach:

You currently have 25 "parts" in your gallon: 1 "part" of oil and 24 "parts" of gas.

You need to add 16 more "parts" of gas. By our definition a "part" is 1/25 of a gallon. You need to add 16 of these, so 16*1/25 = 16/25 = .64 gal.