- There are 2 ways to have a four letter word with the same letters (SSSS, IIII).
- 3 same letters is and 1 different letter is since there are 3 letters different from the same letters. The multiplier takes care of the fact that you could have 4 arrangements as in SSSP, SSPS, SPSS, PSSS.
- 2 same letters is (i.e. there are three different letters that you can have two of). Again, the is a multiplier to include the arragements
- Can you continue with this logic to understand the rest?
Hope this helped.