Math, mainly algebra. So I could've studied more and possibly have gotten a degree and been more successful. I have yet to see in the real world where I'm adding and subtracting some damn letters, and yet that shyt seemed to be important.
You do it all the time without thinking. Say for instance, you're thinking about getting a part time job, you want to work 20 hours a week , and you need to make $500
X = Hourly Rate
20 = Number of hours
Formula would be 20*X = 500, X = 500/20, X=$25
Of course this is just a simple example, but I don't think people realize how often they use algebra, statistics, geometry and calculus on a regular basis.