Level 9-B is a classic mental arithmetic problem, where you're given the end goal of earning a full 13337 keys. And I would fully expect someone to pull out a calculator, or at least a notepad, to do the complicated divisions. Well, I usually do anyways, but I've been challenging myself by attempting to solve the puzzles completely in my head.
booyah
For anyone who doubts, I would be happy to "show my work". I would love to know if you too were able to solve this level completely in your head.