Question:
In how many ways can 4 letters be posted in 5 letter boxes?
Solution:
Given: We have 4 letters and 5 letter boxes
To Find: Number of ways of posting letters.
One letter can be posted in any of 5 letter boxes.
We have to assume that all the letters are different.
So for first letter i.e. $L_{1}$, we have 5 ways
Similarly for,
$L_{2}=5$ ways
$L_{3}=5$ ways
$L_{4}=5$ ways
Total number of ways $=5 \times 5 \times 5 \times 5=625$
In 625 ways 4 letters can be posted in 5 letter boxes.