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

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 $\frac{4!}{2!}$ ways. Hence, required number of arrangements

=$\frac{4!}{2!}=\frac{4\times 3\times 2!}{2!}=12$.