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Printjmc
number theory senior
Problem
How many four-digit numbers whose digits add up to are divisible by ?
Solution
For a number to be divisible by , we need to be divisible by . If the digits of add up to , then must be , because cannot be as large as 11 or as small as without having .
Now implies that , which in turn implies that and have the same parity (that is, they are either both odd or both even). Therefore, is even and therefore cannot be equal to . So there are possible numbers.
Now implies that , which in turn implies that and have the same parity (that is, they are either both odd or both even). Therefore, is even and therefore cannot be equal to . So there are possible numbers.
Final answer
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