Could you help me with question 9 on page 38 please? I don’t understand how to solve it even using back-solving

Yeah, this one is pretty tough! 🙂

To backsolve it successfully, it’s helpful to also plug in. Say that the base and height of triangle A are both 10. Then if you try, say, answer choice C, which says that p = 23, you know that r = 18, and therefore that the base of triangle B is 12.3 (23% more than 10) and the height is 8.2 (18% less than 10). Does that give you the same area for both triangles?

Not quite, but you’re close! For that reason, probably makes sense to try the next closest choice, D. Sure enough, that one works.