Description: A troll forces us to use algebra to figure out the make-up of his currency. We end up setting up a system of equations. Created by Sal Khan. And he says, well, I'm a reasonable troll. And when you look a little bit more carefully, you see that there actually was a sign there that says $5 toll to cross the bridge. And so the troll says, well, you can't cross.*Teacher Advisor is 100% free.