There were eighteen people seated on the roller coaster ride. for every two people seated there was one empty seat. how many empty seats are there?

9 seats were empty between the two people seated of the 18.

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.

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