Final answer:
To find the number of different 9-letter permutations that can be formed from 7 identical H's and 2 identical T's, we can use the concept of permutations.
Step-by-step explanation:
To find the number of different 9-letter permutations that can be formed from 7 identical H's and 2 identical T's, we can use the concept of permutations.
First, we need to calculate the total number of permutations, which is given by 9! (9 factorial), since there are 9 letters in total.
However, since there are 7 identical H's and 2 identical T's, we need to divide the total number of permutations by the permutations of the identical letters. The number of permutations of the H's is given by 7!, and the number of permutations of the T's is given by 2!.
Therefore, the number of different 9-letter permutations is given by:
9! / (7! * 2!)