The number of six letter words that can be formed using the letters of the word "ASSIST" in which S's alternate with other letters is

(a) 12

All S's can be placed either at even places or at odd places, i.e. in 2 ways.

The remaining letters can be placed at the remaining places in 3!, i.e. in 6 ways.

$\therefore$ Total number of ways $=6 \times 2=12$