SOLUTION: Find a formula of the number of permutations of the set {1,2,3,,,,n} where exactly k numbers are in their natural position if you could help me please

Question 984075: Find a formula of the number of permutations of the set {1,2,3,,,,n} where exactly k numbers are in their natural position if you could help me please

Answer by Edwin McCravy(20054) (Show Source): You can put this solution on YOUR website!

We can choose the k numbers to be in their natural positions nCk ways.
The other n-k numbers are not in their natural positions in !(n-k) ways.

Answer (nCk)*!(n-k)

Where !(n-k) = greatest integer not exceeding (n-k)!/e  where e = 2.718281828...

Edwin

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