3. Find the sum of all four-digit numbers in which the digits $0,4,5,9$ do not appear.
Answer: $9999 \cdot 6^{4} / 2=6479352$.

Solution. All four-digit numbers that meet the condition of the problem are divided into pairs of numbers of the form (1111,8888$),(1112,8887),(1113,8886)$, $\ldots$... (4555,5444$)$. In each pair, the sum of the numbers is 9999. Let's count the number of pairs. In total, there are $6^{4}$ four-digit numbers of the specified type (since each digit can be chosen in 4 ways). Therefore, there are half as many pairs. The sum of all numbers is $9999 \cdot 6^{4} / 2=6479352$.

Grading. 12 points for a correct solution. The answer can also be presented as the formula $9999 \cdot 6^{4} / 2$. If the formula is correct but there are arithmetic errors in the calculation using this formula, 10 points.