Question:
In how many ways can 3 letters can be posted in 2 letterboxes?
Solution:
Let Suppose Letterbox be $B_{1}, B_{2}$ and letters are $L_{1}, L_{2}, L_{3}$
So $L_{1}$ can be posted in any 2 letterboxes $\left(B_{1}, B_{2}\right)$ by 2 ways
Similarly, $L_{2}$ can be posted in any 2 letterbox $\left(B_{1}, B_{2}\right)$ by 2 ways
Similarly, $L_{3}$ can be posted in any 2 letterbox $\left(B_{1}, B_{2}\right)$ by 2 ways
So total number of ways is $2^{3}=8$