These are very common mathematical practices at a high school level. I specifically used math that was not calculus based or college level. For this to be "mathemagics" would be to say you don't agree with math that literally forms the basis for all of our civilization. Bridges use far more difficult math than this to construct, how can you regent such basic and well known mathematics that has been valued and trusted for literally millennia. Romans had this math, Greeks had this math, even the ancient Egyptians has some of these techniques.