This question comes from my unsuccessful attempt to solve
(Reason im posting a new thread: dont want to hijack the other one with my own questions).
The question was:
"there are n letters and n addressed envelopes. Letters are placed randomly in envelopes. find the probability that at least 1 letter goes in the correct envelope"
The correct answer is
But i thought it was
So, i have 2 questions:
1) What's the conceptual mistake in my approach
2) Is there a derivation of the subfactorial function somewhere?