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A street has 100 buildings numbered from 1 to 100. How many times is the digit 9 used in numbering these buildings?
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Solution: Step 1: Identify all occurrences of the digit 9 in the units place: 9, 19, 29, ..., 99. That's 10 occurrences. Step 2: Identify all occurrences of the digit 9 in the tens place: 90, 91, ..., 99. That's 10 occurrences. Step 3: Sum the occurrences from both steps: 10 (units place) + 10 (tens place) = 20 occurrences. Step 4: Therefore, the digit 9 is used 20 times.
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