How many zeros, ones, threes? All integers from 1 to 100 are written in a row. How many times do the digits appear: a) zero? b) one; c) three?
Let's record the solution in a table (we write down the number of digits in the corresponding decade).

|  | Counting zeros | Counting ones | Counting threes |
| :---: | :---: | :---: | :---: |
| From 1 to 10 | 1 | 2 | 1 |
| From 11 to 20 | 1 | 10 | 1 |
| From 21 to 30 | 1 | 1 | 2 |
| From 31 to 40 | 1 | 1 | 10 |
| From 41 to 50 | 1 | 1 | 1 |
| From 51 to 60 | 1 | 1 | 1 |
| From 61 to 70 | 1 | 1 | 1 |
| From 71 to 80 | 1 | 1 | 1 |
| From 81 to 90 | 1 | 1 | 1 |
| From 91 to 100 | 2 | 2 | 1 |

## Answer

a) 11; b) 21; c) 20.