Alberto is learning to write the numbers at school. Today the teacher has made them write all the numbers from one to 100.
Do you know how many times you will have written the digit 3?
In total he will have written 20 times the number 3. Once for every ten except 2 times for 33: 3, 13, 23, 33, etc. which are 11 in total and 9 times more from 30 to 39, not counting again 33.