In how many ways 6 letters can be put in 6 addressed envelopes so that only 3 of them will go in the correct envelopes?
40
Here, we have to select 3 letters out of 6
Number of ways is 6C3
We should de arrange the 3 letters.
Required number of ways = 6C3×3!(1−11!+12!−13!)
=40