6. (6 points) On the blackboard, there is a number in the form of $7777 \cdots 77$. Each time, erase the last digit, multiply the remaining number by 3, and then add the digit that was just erased. Continue operating on the new number in the same way, and the final number obtained is . $\qquad$
【Answer】Solution: According to the above analysis:
Since $7777 \cdots 77$ can be divided by 7, the last digit must be 7. Therefore, the answer is: 7.