It seems to me that the following rules apply:
- When the answer is a one-digit number, the character 如 is used.
- When the answer is 10, the character 得 is used.
- When the answer is a multiple of 10, other than 10 itself, the word 中 is used.
- When it is 3×3=9, the character 歸 is sometimes used (though not on this picture here).
- In all other cases, no additional character is inserted.
I am wondering why there are such rules? Do we use these rules anywhere else?