Envelopes
Probability
Level
2
Ten letters are put in 10 addressed envelopes .Find the number of ways in which only nine of the letters are put in the correct envelopes.
The answer is 0.
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It is obvious placing 1 thing in a wrong way cannot be done since it has to be placed in others place so other has to be put in wrong place also. This is also used in proof of derangement theorem mathematical formula.