How many words can be formed from the letters of the word SERIES, which start with S and end with S?

Explanation

In the word 'SERIES' there are `6` letters of which `2` are S and `2` are E's.


Let us fix `5` at the extreme left and at the extreme right end. Now, we are left with `4` letters of which `2` are E's.
These four letters can be arranged in `(4!)/(2! )`ways. Hence, required number of arrangements

=`(4!)/(2!)=12`.