Total number of ways in which the letters can be put = 3! = 6

Suppose, out of the three letters, one has been put in the correct envelope.

This can be done in ³C₁ ways. (3 ways)

Now, out of three, if two letters have been put in the current envelope, then the last one has been put in

the correct envelope as well. This can be done in ³C₃ ways. (1 way)

Number of ways = 3 + 1 = 4

Number of ways in which no letter is put in correct envelope = 6 – 4 = 2