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jmc

number theory senior

Problem

A positive five-digit integer is in the form ; where , and are each distinct digits. What is the greatest possible value of that is divisible by eleven?
Solution
We can test an integer for divisibility by by alternately adding and subtracting its digits. For example, is divisible by 11 because is divisible by 11. In this case, must be divisible by 11. If there are satisfactory values of and corresponding to , then the resulting integer would be larger than any integer with . Therefore, we try first. If , then must be divisible by . Equivalently, equals or , which implies or . Wanting to make as large as possible, we try . cannot be because , , and must be distinct. If , then , so again the digits are not distinct. If , then and still the digits are not distinct. If , then , and .
Final answer
96,\!569