If 5 letters are taken out of 5 different envelopes. In how many ways, can they be reinserted in the envelopes so that no letter goes to its original envelopes?
44
Number of possible de arrangement that are possible out of n letters in n envelopes.
The number of ways= Dn=n!∑k=0n(−1)kk!
Number of possible de arrangements that are possible out of 5 letters in 5 envelopes
=5!(1−11!+12!−13!+14!−15!)
=44