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.
72 / 9 = 8
8 / 72 = 1/9
1/9 / 8 = 1/72
1/72 / 1/9 = 1/8