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

Answer: Option A

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 (up-to 10 times).
= 5¹⁰