I remember when I joined the site there were practice problems in which you were awarded stars on solving a set. You were also awarded more problem sets if you invited your friends. They included all kinds of math problems sorted by topics, from algebra and number theory to calculus. The three levels were Beginner, Intermediate and Advanced. I did not pay any heed to that for months but when today I was searching for them, I couldn't find them. I remember some of the problems were really awesome. Does anyone know what happened or it's simply that I can't find the link to them. The same thing goes for notes on various things about maths including factorization, euclidean algorithm and high school math. Where are they?
Any help, respected Staff?