8-7. Petya thought of four different digits, not equal to 0. Then he formed all possible four-digit numbers from these digits without repeating any digits. The sum of all these numbers turned out to be 73326. What 4 digits did Petya think of?
Answer: $1,2,3,5$.

Solution. Let Petya have thought of the digits $a, b, c$ and $d$. In total, there will be 24 numbers composed of these digits. If we add all these numbers, each of the digits $a, b, c$ and $d$ will appear in each place value 6 times. Therefore, the sum of all 24 numbers will be equal to $6 \cdot(a+b+c+d) \cdot 1111=73326$. From this, $a+b+c+d=11$. The number 11 can be represented as the sum of four different digits in only one way: $11=1+2+3+5$. These are the digits Petya thought of.