3.1 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 all numbers from 2000 to 2021.

Natural numbers are numbers used for counting objects.

Answer: 20012013456789
Solution. The number must definitely contain three zeros (for the number 2000), two ones (for the number 2011), and two twos (for the number 2020). Other digits must appear at least once, so the result should have a minimum of 13 digits. Before the third zero, there must be both twos (for the number 2020) and at least one of the ones (2010). The remaining digits can then be arranged in ascending order.