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.