13.168. A one-digit number was increased by 10 units. If the resulting number is increased by the same percentage as the first time, the result is 72. Find the original number.
Solution.

Let $x$ be the number we are looking for, which was increased by $y \cdot 100\%$. Then $x+10=y x$, from which $y=\frac{x+10}{x}$. According to the condition $(x+10) \cdot \frac{x+10}{x}=72$, from which $x=2$.

Answer: 2.