2. When writing a decimal fraction, a schoolboy wrote all its digits correctly, but placed the comma in such a way that the resulting fraction was less than the required one by 18.189. What decimal fraction should the schoolboy have written?
Solution: Since the number decreased as a result of the error, the decimal point must have been moved to the left. This means the number was reduced by a factor of 10. Let the resulting number be x, then the original number is 10x. According to the problem: $10x - x = 18.189$, so $x = 2.021$, and $10x = 20.21$.

Answer: $20.21$.