How many digits are needed to write down the numbers from 1 to 6786?
Among the numbers ranging from 1 to 6786, there are 9 single-digit, 90 double-digit, 900 triple-digit, and 5687 four-digit numbers. Therefore, to write these down,

$$
9 \cdot 1 + 90 \cdot 2 + 900 \cdot 3 + 5687 \cdot 4 = 26037
$$

digits are required.

(József Löwy, Losoncz.)

Number of solutions: 36.