Question:
How many words can be formed from the letters of the word ‘SERIES’, which start with S and end with S?
Solution:
To find: number of words which start and end with S
There are 4 places to fill up with 4 letters out of which 2 are of the same kind
$\Rightarrow$ Number of words $=\frac{4 !}{2 !}=12$
12 words are possible