6. (5 points) Given that the average of 9 numbers is 9, if one of the numbers is changed to 9, the average of these 9 numbers becomes 8, then the number that was changed was originally $\qquad$ .


【Analysis】The total before the change is $9 \times 9=81$, and the total after the change is $8 \times 9=72$. The difference before and after is $9 \times 9-8 \times 9$ $=9$, which indicates that this number has decreased by 9. Therefore, the original number that was changed is $9+9=18$; solve accordingly.

【Solution】Solution: $9 \times 9-8 \times 9$
$$
\begin{array}{l}
=81-72 \\
=9 \\
9+9=18
\end{array}
$$

Answer: The original number that was changed is 18.
Therefore, the answer is: 18.