I had to complete integral calculus, which was a PITA. Granted, I didn't do well in math in High School. I was too busy chasing skirts and drinking beer. When I returned to college several years later, I started out at "dummy math", which was basic addition, subtraction etc, then took math every single semester/quarter (2 different schools on different systems) until I completed IC. Some I took and failed in HS, but again, wasn't interested at that point. It really builds one on the other, and when you can take it continuously, without significant breaks, it actually is easier, because you never get "out of the habit" of doing math. I now find that I'm using stuff I'd forgotten on a frequent basis.
Erik