Each number comes from multiplying the one before it by 1/3 .
The ones that are already printed there show this rule:
The first one is 9.
Then . . .
9 x 1/3 = 3
3 x 1/3 = 1
1 x 1/3 = 1/3
1/3 x 1/3 = 1/9
The next one is 1/9 x 1/3 = 1 / 27
The one after that is 1/27 x 1/3 = 1 / 81
And you ought to be able to do the last one on your own by now.