9.2. All natural numbers from 1 to 1000 inclusive are divided into two groups: even and odd. In which group is the sum of all digits used to write the numbers greater, and by how much?
Answer. The sum of the digits of odd numbers is 499 more.

Solution. The sum of the digits of the number 1 is equal to the sum of the digits of the number 1000; the other numbers can be divided into pairs: $2-3,4-5,6-7,8-9, \ldots, 998-999$. In each pair, the unit digit of the odd number is 1 more than that of the even number, while the tens and hundreds are the same. There are 499 such pairs.