Maths Olympiad Prep

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Combinatorics Difficulty 5.6 AIME, harder Prove it Japan

Find the average value of all those integers nn satisfying 0n100000 \le n \le 10000 for which the digit 11 does not appear in their decimal expansions.

Solution

Call a non-negative integer less than or equal to 1000010000 for which the digit 11 does not appear in its decimal expansion a good integer.
Let us denote by AA the average value of all good integers. Since 1000010000 is not a good integer, it suffices to consider only good integers of 44 or less digits. Write a good integer in the form a1+10a2+102a3+103a4a_1 + 10a_2 + 10^2a_3 + 10^3a_4, where aja_j's are chosen from the set {0,2,3,4,5,6,7,8,9}\{0, 2, 3, 4, 5, 6, 7, 8, 9\}.
Since for a1a_1 each of the values 0,2,3,,90, 2, 3, \ldots, 9 can be chosen the same number of times, the average value for a1a_1 over the set of all good integers is 19(0+2+3+4+5+6+7+8+9)=449\frac{1}{9}(0 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9) = \frac{44}{9}.
Similarly, the average value of a2,a3,a4a_2, a_3, a_4 over the set of all good integers is 449\frac{44}{9} for each one. Hence we get
A=449(1+10+102+103)=488849.A = \frac{44}{9}(1 + 10 + 10^2 + 10^3) = \frac{48884}{9}.

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