.We need to find the probability that atleast one letter was put in the correct envelope.

The total number of ways of putting three letters in 3 envelopes = 3 x 2 x 1 = 6

The number of ways of putting all three letters in incorrect envelope is =2

Letter for X goes into envelope for Y and Letter for Y goes into envelope for Z, Letter for Z goes

into envelope for X

or Letter for X goes into envelope for Z and Letter for Y goes into envelope for X and Letter for Z goes into

envelope for Y ]

2 1

P(None of the letters is put in the right envelope) =

6 3

P(Atleast one letter is put in the right envelope)=

=

1 2

1 _ - P(None of the letters is put in the right envelope) 1

3 3