The "%" sign is Shift-5. Option-5 gives the leminscate, otherwise known as the sign for infinity: "∞"
It would make sense for Option-Shift-5 to be related to a number, but instead it's "fi"
No, I did not just type "f" then type "i." Oddly enough, Option-Shift-6 gives you "fl"
How in the world did the R-key get roped into percentages? Option-R is the registered trademark symbol (®), which makes sense since it contains, y'know, an R.
Yes, I understand the concept of typographical ligatures, but what makes fl so special? We could have had a few more Greek letters instead.
