I agree! It is very challenging. As I said before, there were several problems on the Challenge Set of Intro to Alg AoPS that DS9 couldn't do. So I tried, DS13 (in Alg 2--math student of the year for his high school) tried, and my engineer-background father-in-law tried. Very challenging. No one could get, except me after thinking about it day & night for a couple days.