Find the number of four letter words which can be made using the letters of the word "HIPPOPOTAMUS", if not all four letters are different.

Explanation

To solve the problem of finding the number of four-letter words that can be formed from the letters of "HIPPOPOTAMUS," where not all four letters are different, we will break the problem down into cases based on the repetition of letters.

### Step-by-Step Solution:

1. **Identify the Letters**:
The word "HIPPOPOTAMUS" consists of the following letters:
- Unique letters: H, I, T, A, M, U, S (7 unique letters)
- Repeated letters: P (3 times), O (2 times)
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