I haven’t taken a math class in a year or two, so my math is a little rusty. But, the two things I remembered are 1. Quadratic equations, and 2. Polynomials. Both I believe have 2 or 3 methods and answers (and the answers may vary depending on how you solve them - I think). I learned these in like, my sophomore or junior year of high school, but it may be taught earlier in other places, so idk if either would work for your story.