3.2. Let's say that number A hides number B if you can erase several digits from A to get B (for example, the number 123 hides the numbers 1, 2, 3, 12, 13, and 23). Find the smallest natural number that hides the numbers 121, 221, 321, ..., 1921, 2021, 2121.

Natural numbers are numbers used for counting objects.

Answer: 1201345678921
Solution: The number must contain one zero, three ones (for the number 1121), two twos (for the number 1221), and at least one of each of the other digits, meaning it must have a minimum of 13 digits. After two ones and the other digits, one two and one one stand consecutively, so they will be at the end. The smallest digit the number can start with is one. To the left of the second two is one one, and to the right are two ones (in the numbers 2121 and 1221). The zero stands to the right of the two and the one (in the numbers 2021 and 1021), so the second digit is 2, and the third is 0. Then, one one can be placed, followed by the remaining digits in ascending order.