7.4. Seryozha wrote a five-digit natural number on the board. It turned out that in this number, among any two adjacent digits, the one on the right is greater. Vasya multiplied this number by 9 and calculated the sum of the digits of the resulting product. Find all possible values that this sum can take. Justify your answer.
Solution: Let Sergey write the number $x=\overline{a b c d e}$. Then $10 x=\overline{a b c d e 0}$. Notice that $9 x=10 x-x$ and perform the subtraction "in columns":

![](https://cdn.mathpix.com/cropped/2024_05_06_9c478154f2d203c4a23fg-5.jpg?height=265&width=945&top_left_y=136&top_left_x=561)

Due to the condition $a$, the number in the answer is no more than 45, and it is possible that the correct answer is absent | 3 points |
| The correct answer is provided, supported by examples (in any quantity), but the uniqueness of the answer is not proven | 1 point |
| Answer without justification and/or incorrect answer | 0 points |