I don't remember learning this

I always have to add mailboxes in everyday life.

Oh it's easy. Y is equal to doorknob

Actually, the mailbox would be treated like any other variable (which is just an unknown number represented by a symbol, though usually a letter). The final answer would be 13/72x+[mailbox], since you can't add different variables together.

Someone had too much fun with Windings at the textbook company.