In how many ways can 4 letters be posted in 3 letter boxes?

Explanation

Since each letter can be posted in any one of the three letter boxes. So, a letter can be posted in 3 ways.
Since number of letters = 4 and each letter can be posted in 3 ways
`:.` Total number of ways in which all the 4 letters can be posted `= 3 xx 3 xx 3 xx 3 = 3^(4) = 81`.