How Many Times Paint Number 9

John has been hired to paint the numbers 1 through 100 on 100 apartments. How many times will he have to paint the number 9?

Answer

20 times. They are 9, 19, 29, 39, 49, 59, 69, 79, 89, 99 (2 times) followed by 90, 91, 92, 93, 94, 95, 96, 97, 98.