Given are 111 distinct positive integers not exceeding 500. May it happen that for each of these numbers, its last digit coincides with the last digit of the sum of all other numbers?
Solution
Suppose this is possible. Denote the given numbers by and let their sum be . By the condition, for each index , the numbers and have the same last digit. Hence, their difference is divisible by . Therefore, for any , the number ends with the same digit as the sum . This means that the difference between any two numbers is divisible by . Thus, all distinct numbers must give the same remainder when divided by ; but among the numbers from to , there are exactly numbers with any fixed remainder modulo . Contradiction.
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