Fifteen people are sitting at a round table. There are 15 name cards on the table, but no one is sitting in front of their own name card. Show that you can rotate the table in such a way that at least two persons will sit in front of their own name card. How do I show this?
I know that you have to use the pigeonhole-principle, but I can't seem to understand in which way I can use this principle in this case.