In how many ways can 3 letters can be posted in 2 letterboxes?
Let Suppose Letterbox be B1, B2 and letters are L1, L2, L3
So L1 can be posted in any 2 letterboxes (B1, B2) by 2 ways
Similarly, L2 can be posted in any 2 letterbox(B1, B2) by 2 ways
Similarly, L3 can be posted in any 2 letterbox(B1, B2) by 2 ways
So total number of ways is 23 = 8