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The answer here should be 4!, which is, in English, either 4 factorial or 4 shriek, which is 4 x 3 x 2 x 1. The logic behind the answer is as the following.
We are trying to create 4-letter words with 4 different letters, M, A, T and H. Consequently, there are 4 possible letters this 4-letter word can start with. Next, we consider the second letter, since one letter has been taken, there remains only three possibilities. Similarly, only 2 letters remain to choose the third letter, and there is no choice for the fourth letter. So multiplying 4 x 3 x 2 x 1 gives 4!.