Board Thread:General Discussion/@comment-25143095-20140730174850

especially when you have exercises from hell like:

https://www.khanacademy.org/math/algebra/linear-equations-and-inequalitie/equations-modeling/e/modeling-with-two-variable-equations-and-graphs

IMHO - Mastering all of the excercises in the World of Math is significantly harder and a huge achievement. 