If the letters of the word ‘MISSISSIPPI’ are written down at random in a row, what is the probability that four S’s come together.
Letters of MISSISSIPPI has 11 letters. Out of which 4 S’s are repeating, 4 I’s are repeating and 2 P’s are also repeating.
By Permutation theory.
Letters of MISSISSIPPI can be arranged in 
Take 4 S’s together and treat them as single letter. Now we have
8 letters in together – MI(SSSS)IIPPI → MI(X)IIPPI
Letters of MI(X)IIPPI can be arranged in 
∴ P(4’S come together) = 
6