Look up Derangements, for example here, to answer (b).
1. Six different letters are to be put into 6 addressed envelopes.
a) In how many ways can this be done?
b) In how many ways can letters be put into envelopes with the wrong address?
2. In how many ways can 10 chairs be allocated to three rooms with 3 in the first, 5 in the second, and 2 in the third?
I know you are only trying to help rgep, but I still don't understand how to apply the derangements example to my question, I'm sorry and so I have no idea how to answer any of these. I realise that my thread was written almost a week ago, but if anyone is reading through this, I really need some help on this. Thank you.