Answer: 0
Explanation:
First you do 1 through 9,
then 10 through 19,
then 20 through 29.
You have one set of 9 numbers,
1-9, and 9 sets of ten numbers such as 20-29. Looking at the tens digit, 1 is used 10 times for 10-19, 2 is used 10 times for 20-29, 3 is used 10 times, etc.
However, 0 is not used as a tens digit except in the number 100.
If the digit is not 0, then it occurs 10 times as a ten digit.
In addition, it occurs once in each set of ten, for a total of 20 times. For example, consider 6. It occurs ten times in 60-69, and then another ten times as 6, 16, 26, 36, etc., for a total of 20.
O occurs as 10, 20, 30, 40, 50, etc plus an extra 0 in 100, for a total of 11 times.