6. Petya formed all possible natural numbers that can be formed from the digits $2,0,1$, 8 (each digit can be used no more than once). Find their sum. Answer: 78331
Solution: First, consider the units place. Each of the digits 1, 2, 8 appears once in this place for single-digit numbers, twice for two-digit numbers, four times for three-digit numbers, and four times for four-digit numbers - a total of 11 times.

In the tens place, each of them appears 3 times for two-digit numbers, 4 times for three-digit numbers, and 4 times for four-digit numbers - also 11 times.

In the hundreds place, each appears 6 times in three-digit numbers and 4 times in single-digit numbers. In the thousands place, each appears 6 times.

In total, we get $11 \times 11 + 11 \times 11 \times 10 + 11 \times 10 \times 100 + 11 \times 1000 \times 6 = 78331$.