Answer : 20
Counting from 1 to 100,
To count the number of 6, lets start with
Number of sixes in once place: 6, 16, 26,36,46,56, 66, 76, 86, 96 with a total of 11 sixes
Number of sixes in tens place: 60, 61, 62, 63, 64, 65, 67, 68, 69 with a total of 10 sixes (66 already counted)
11 + 10 sixes = 20 sixes