"Three letters are dictated to three person and an   envelope is addressed to each of them,
Channel: Doubtnut
Date: 2020-07-12

"Three letters are dictated to three person and an
 
  envelope is addressed to each of them, the letters are inserted into the envelopes
 
  at random so that each envelope contains exactly one letter. Find the
 
  probability that at least one letter is in its proper envelope."