It can be understood as follows:

If it is addition, first look at the gap between the first item and 9, and then compare the gap between the second item and the previous result. Greater than it is greater than 9.

For example, 8+3 8 is less than 9 times 1 and 3 is greater than 1, so 8+3 >; nine

If it is addition, first look at the gap between the first item and 9, and then compare the gap between the second item and the previous result. If it is greater than it, it is less than 9.

For example, 18- 12 18 is greater than 9 degrees, so18-12.