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Printjmc
number theory intermediate
Problem
If the seven-digit number is divisible by , what is ?
Solution
A number is divisible by if and only if the sum of the first, third, fifth, etc., digits less the sum of the second, fourth, sixth, etc., digits is itself a multiple of . The former sum is . The latter sum if . Thus must be a multiple of . This is satisfied only by .
Final answer
5