Find the average value of all those integers satisfying for which the digit does not appear in their decimal expansions.
Solution
Call a non-negative integer less than or equal to for which the digit does not appear in its decimal expansion a good integer.
Let us denote by the average value of all good integers. Since is not a good integer, it suffices to consider only good integers of or less digits. Write a good integer in the form , where 's are chosen from the set .
Since for each of the values can be chosen the same number of times, the average value for over the set of all good integers is .
Similarly, the average value of over the set of all good integers is for each one. Hence we get
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