You are definitely judging the book by its cover. If you follow the link, the full book has a lot more "tricks" and "solutions". The cross-multiply one is just

You are definitely judging the book by its cover. If you follow the link, the full book has a lot more "tricks" and "solutions". The cross-multiply one is just an example. Of course many of them are culture/location dependent, but I'm sure you will find ones that you are familiar with.