The next number in the series is always found by dividing the previous number by the number before it. So you calculate the number at position 3 by dividing the number at position 2 by the number at position 1.
18 / 2 = 9
9 / 18 = 1/2
1/2 / 9 = 1/18
1/18 / 1/2 = 1/9
1/9 / 1/18 = 2
2 / 1/9 = 18