3. Find the sum of all four-digit numbers in which the digits $0,3,6,9$ 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), ..., (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.