There is 9 empty seats because if you put it like this-
($- are people)( Empty Seat [])
$ $ []
$ $ []
$ $ []
$ $ []
$ $ []
$ $ []
$ $ []
$ $ []
$ $ []
So if there is 2 people in each row there is one empty seat... so for every 2 people there is one empty seat. So you have 9 empty seats for the 18 people riding.
