【Analysis】If the original number is even, then the last digit that is erased must be even, and the number after multiplying by 4 is also even, and adding it is also even, so the final result must be even. If the original number is divisible by 3, let the original number be $10 a+b$, where $b$ is the last digit, then after one transformation it becomes $4 a+b$, and the difference after two transformations is $9 a$, which indicates that if the original number is a multiple of 3, then the later number is also a multiple of 3.

In summary, the final result can only be 6.
【Solution】If the original number is divisible by 3, let the original number be $10 a+b$, where $b$ is the last digit, then after one transformation it becomes $4 a+b$, and the difference after two transformations is $6 a$, which indicates that if the original number is a multiple of 3, then the later number is also a multiple of 3. Since it is even, this single-digit number is 6.

Therefore, the answer is: 6.
20. (6 points) On the blackboard, there is a nine-digit number 222222222. Perform the following operation: erase the last digit, then multiply by 4, and add the digit that was just erased, and write down the result on the blackboard; continue this process until a single-digit number is written on the blackboard. What is that number? $\qquad$