Since there are 9 letters, there are 9! ways to arrange them. However since there are repeating letters, we have to divide to remove the duplicates accordingly. There are 2 ‘s’ and 2 ‘i’ hence:
Number of way to arrange ‘dissipate’ = 9! / (2! x 2!) = 90720 ways
Hence there are 90720 ways to have the number of dissipate in the letter.
