Compute the number of positive four-digit multiples of 11 whose sum of digits (in base ten) is divisible by 11.
Solution
Let an arbitrary such number be . Then, we desire and , where the latter comes from the well-known divisibility trick for 11. Sums and differences of multiples of 11 must also be multiples of 11, so this is equivalent to desiring and . As and , and must be either 0 or 11 (no larger multiple is achievable). There are 8 choices for such and 9 choices for such , so the answer is .
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