| Dima writes natural numbers in a row: 123456789101112... .

On which positions, counting from the beginning, will the three digits 5 appear in a row for the first time?
Before the number 50 was recorded, the digit 5 only appeared in the units place, so there could not have been two fives in a row.

Three digits 5 will first appear in the sequence ...54-55-56... . From 1 to 9, there are 9 digits, and from 10 to 54, there are 45 two-digit numbers, which means there are a total of 99 digits before the first required digit 5. Therefore, the three digits 5 are on the $100-\mathrm{th}, 101-\mathrm{th}$, and $102-\mathrm{th}$ positions.

## Answer

On the $100-\mathrm{th}, 101-\mathrm{th}$, and $102-\mathrm{th}$ positions.