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The post is not absolutely clear to be considered as a Math problem.
Let me explain why.
The given word "DEPARTMENT" has two identical letters "E" and two identical letters "T".
One part of the condition assign me to use each letter at most once.
The other part says "each letter must be used".
Under these instructions, I don't know if I can and if I should use the letter "E" twice.
Same about letter "T".
Also, should I consider permutations of two different instances of "E" between themselves inside a word as giving different words ?
As you, probably, understand, a TRUE MATH problem should not leave a room for such doubts.