SOLUTION: in how many way can 10 letter be put in 10 addressed Envelope such that exactly 5 letter go in wrong envelopes.

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Question 935769: in how many way can 10 letter be put in 10 addressed Envelope such that exactly 5 letter go in wrong envelopes.
Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
in how many way can 10 letter be put in 10 addressed Envelope such that exactly 5
letter go in wrong envelopes.
We can choose the 5 letters to be placed in the correct envelopes C(10,5)
ways and for each of those ways, there are !5 (5 sub-factorial) ways to
place the other 5 wrong.

Answer: C(10,5)*!5 = expr%2810%2A9%2A8%2A7%2A6%2F%285%2A4%2A3%2A2%2A1%29%29%2Aexpr%28Int%285%21%2Fe%29%29+=+11080, where Int() is the greatest integer function.

Edwin