How many ways can 10 letters be posted in 5 post boxes, if each of the post boxes can take more than 10 letters ?

Each of the 10 letters can be posted in any of the 5 boxes.

So, the first letter has 5 options, so does the second letter and so on and so forth for all of the 10 letters.

i.e. 5*5*5*….*5 (upto 10 times) = 5 ^ 10.