If all letters of the word MISSISSIPPI are rearranged then the probability that all S come together will be

There are 11 letters in the given word containing four S, four I, two P and one M.
Therefore total number of words $=\frac{11!}{4!4!2!}$

Hence when four S come together then they may start from first letter, from second letter, ....., from eighth letter. In each of such cases the remaining seven letters may occur in $\frac{7!}{4!2!}$ ways.

Hence total number of arrangements in which four S come together $=8\times \frac{7!}{4!2!}$

$\therefore$ Required probability $=\frac{{\displaystyle \frac{8.7!}{4!2!}}}{{\displaystyle \frac{11!}{4!4!2!}}}=\frac{4}{165}$