49. Sergei wrote down a certain five-digit number and multiplied it by 9. To his surprise, he got a number consisting of the same digits but in reverse order. What number did Sergei write down?
49. Since multiplying a five-digit number by 9 results in a five-digit number, the leftmost digit of the original number is 1.

Since the number obtained after multiplying this number by 9 ends with the digit 1, the original number ends with the digit $9(9 \cdot 9=81)$

![](https://cdn.mathpix.com/cropped/2024_05_21_00604dc020e3721fc1f4g-128.jpg?height=143&width=223&top_left_y=1319&top_left_x=925)

Since the second digit after 1 can only be 0, then

![](https://cdn.mathpix.com/cropped/2024_05_21_00604dc020e3721fc1f4g-128.jpg?height=142&width=240&top_left_y=1528&top_left_x=908)

Since the second digit in the original number is 8, then

$$
\begin{array}{r}
10 * 89 \\
\times \quad 9 \\
\hline 98 * 01
\end{array}
$$

Since the third digit in the original number is 9, then

10989

$$
\times \frac{9}{98901}
$$