There are 7 letters in fission, which we'll use as a factorial in the numerator. Of those 7 letters, there are two I's and two S's. To account for these repeating letters, we'll use the factorials of how many times the letters repeat and plug them into the denominator.
The factorial of 7 should be your numerator, and two factorials of 2 should be your denominator:
[tex] \frac{7!}{2! \times 2!} = \frac{5040}{2 \times 2} = \frac{5040}{4} = 1260[/tex]
There are 1260 different permutations of the word fission.
