Luis wrote the sequence of natural numbers, that is,

$$
1,2,3,4,5,6,7,8,9,10,11,12, \ldots
$$

When did he write the digit 3 for the $25^{\mathrm{th}}$ time?
Let's look at all the times Luis wrote the digit 3:

- $3 \sim 1$;
- $\underbrace{13,23}_{2}, \underbrace{30,31,32,33, \ldots, 39}_{11}, \underbrace{43, \ldots, 93}_{6} \sim 2+11+6=19$.

So far, he has written the digit 3 twenty times. From there, we have

![](https://cdn.mathpix.com/cropped/2024_05_01_666a1aec7e447b0a6cbeg-088.jpg?height=103&width=599&top_left_y=2490&top_left_x=814)

Therefore, by writing the number 131, he wrote the digit 3 for the $25^{\text {th }}$ time.