Answer:
Add I, add 9, repeat.
Explanation:
It is because if you look at the Add 9 subtract 8, repeat. it won't work
If you look at add 10, subtract 9, repeat. it its just like the first one, it won't work.
If you look at add 1, add 10, repeat, it also won't work from when you are getting 6 to 15.
So if you look at Add I, add 9, repeat., it works for all.