A certain street contains 100 buildings. They are numbered from 1 to 100. How many 9's are used in these numbers?

20
Just count the nines in the numbers: 9, 19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99.
Note: 99 contains TWO nines!

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