You are missing that there are two s's.
For this question: how many different arrangements of the letters of the word SERIES are possible if no two vowels are together. I keep on getting 36 instead of 18
My working out is:
Consonants can be arranged in 3!/2! ways (X X X) so vowels can be allocated between any of the 4 spaces available i.e. (4 * 3* 2)/2!
What am I missing out??