Problem 4. The numbers from 1 to 100 are written one after another to form a large multi-digit number: 123456789101112131415...979899100.

How many digits does this number have?
Solution. The number of all digits: there are 9 one-digit numbers, there are 90 two-digit numbers and one three-digit number, $9+90 \cdot 2+3=192$ digits.