8. (10 points) Xiaoyuan and Xiaofang are working together on a 10,000-character typing assignment, but they are both very careless. Xiaoyuan makes 1 mistake for every 10 characters he types, and Xiaofang makes 2 mistakes for every 10 characters she types. In the end, when both have completed their work, the number of correct characters typed by Xiaoyuan is exactly twice the number of correct characters typed by Xiaofang. Therefore, the total number of correct characters typed by both is ( ).
A. 5000
B. 7320
C. 8000
D. 8640
【Solution】Solution: According to the problem, for every 10 parts of text Xiao Yuan types, he makes 1 mistake, and for every 10 parts of text Xiao Fang types, she makes 2 mistakes, meaning that when Xiao Fang types 5 parts of text, only 4 parts are correct, and 1 part is correct.

The ratio of correct parts to mistakes for Xiao Yuan and Xiao Fang is $9:4$. Since the total ratio of correct parts is $2:1$, the ratio of mistakes made by Xiao Yuan and Xiao Fang is $\frac{2}{9}:\frac{1}{4}=8:9$.

The number of correct parts typed by Xiao Yuan and Xiao Fang is $(8 \times 9):(4 \times 9)=72:36$.
The total number of parts typed by Xiao Yuan is: $72 \div 9 \times 10=80$ parts.
The total number of parts typed by Xiao Fang is: $36 \div 4 \times 5=45$ parts.
The total number of parts is: $80+45=125$ parts.
The number of characters per part is: $10000 \div 125=80$ (characters).
The total number of correct characters typed by Xiao Yuan and Xiao Fang is: $(72+36) \times 80=8640$ characters.
Therefore, the answer is: $D$.