I actually did that exact same idea and got the 50:1 mixture. But does that hold up with other mixtures? IDK... Math is such an amazing thing...
I do remember my chemistry class where we had to do such type equations. But I since forgotten the formula you used.
A 16/1 ratio would be
768 / 17 = 45.1764
45.1764 x 16 = 722.8224 oz gas
45.1764 x 1 = 45.1764 oz oil
722.8224 + 45.1764 = 767.9988
722.8224 / 45.1764 = 16.0 or 16/1
This is not a equation problem, it is a simple division / multiplication problem. You could do a complex 2 variable equation and spend much more time solving it.
When my kids were in school they would have homework that occasionally I would look at, or help with. There were several times where I would help, providing the correct answer. I could not do the process that the teachers were looking for, but always had the correct answer. My kids would provide the correct answer but get a zero grade because they didn't show the "correct" method of getting the answer.
New Math... Male Cow Manure...