6 letters are to be posted in three letter boxes. The number of ways of posting the letter when no letter box remains empty is?

1 letter has 3 choices to go. In any of the box it can go. So there are total 6 letters so total 3^6 ways in which all three letters can be posted but for the condition that no letter box remian empty we subtract those cases in which box is empty. So total 3 ways in which box will be empty so 3^6-3 is answer