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A release-checked easy problem for training Combinatorics.

Question

How many times does the digit $9$ occur when counting $1$ to $1000$? Note that the number $919$, for example, would count as $2$ $9$s.

Practice focus

This Brainteasers problem is tagged Combinatorics. State the random variables and conditioning information explicitly, then check the result against boundary cases before opening hints or a solution.

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