Originally Posted by

**Leviathantheesper** The problem is:

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Let's Suppose that [FONT=arial]I'm a teacher, I take some notebooks from nine students, and I give them back. ¿How many ways are there to give back the notebooks in a way that no one receives his own notebook?

What I do, naturally, is to reduce the problem, and try making a formula.

I look what happens with four notebooks...I started looking how many ways there are...and I find there are 9 ways. With 3 notebooks there are only 2 ways. And with 2 there is only 1. No easy-looking pattern, so I can't make a formula that easy...