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counting and probability intermediate
Problem
The number has the property that its units digit is the sum of its other digits, that is . How many integers less than but greater than have this property?
(A)
(B)
(C)
(D)
Solution
We take cases on the thousands digit, which must be either or : If the number is of the form where are digits, then we must have Since we must have By casework on the value of , we find that there are possible pairs , and each pair uniquely determines the value of , so we get numbers with the given property. If the number is of the form then it must be one of the numbers Checking all these numbers, we find that only has the given property. Therefore, the number of numbers with the property is .
Final answer
D