10. (6 points) In a two-digit number, add a decimal point in the middle to get a decimal. If the sum of this decimal and the original two-digit number is 86.9, then the original two-digit number is $\qquad$ .
10. (6 points) In a two-digit number, add a decimal point in the middle to get a decimal. If the sum of this decimal and the original two-digit number is 86.9, then the original two-digit number is $\qquad$.

【Solution】Solution: According to the problem, we have:
$86.9 \div(10+1)=7.9$;
$7.9 \times 10=79$.
Answer: The original two-digit number is 79.
Therefore, the answer is: 79.