Total no. of ways of posting = 4!
Now, if one letter is in the correct envelope, then 2nd letter can go into 2 of the 3 remaining envelopes (as the 3rd is the correct envelope), and so on... and there are 4 ways in which this can be done. So, we get
No. of ways only one letter is correctly posted = 4 x (2 x 1 x 1)
Probability = 1/3
i'm sorry i don't understand your answer.
I'm getting 1/4 because say
the first letter.. 3/4 chance it goes into the wrong envelope. 4 letter/ 3 incorrect envelopes
the second letter.. 2/3
the third letter... 1/2
3/4 * 2/3 * 1/2 = 6/24 = 1/4