«Add 7.1% sales tax to the $19.95 ticket» (or whatever) is a 1-step (or generously 2-step) arithmetic problem, of the kind most students in many parts in the world learn to understand conceptually at age 6–7 with no problem. It additionally involves multi-digit multiplication, of the type people typically learn by about age 9–10 (?).

If students have not practiced solving problems enough to handle combining these things until age 13, and many are still struggling with it at age 15 (with a calculator!), that’s a serious indictment of the entire education system.