If you know how much 8 is compared to ten (2 units less than), then you could compare how much 9 is to ten (1 unit less than).
So if you subtract something that is 2 units less than 10 from something that is 1 unit less than ten (9-8), then you will get 1.
Because the numbers have one unit of a difference. (2-1=1)
Hope this answer is what you were looking for ;)
