You have 2007 cards. On each card, a positive integer less than 2008 is written. If you take a number (at least 1) of these cards, the sum of the numbers on the cards is not divisible by 2008. Prove that the same number is written on every card.
Solution
Do not. Let the numbers on the cards be where . Let
for . We now know that for all . Suppose that with means that the non-empty sum is equal to 0 modulo 2008; again a contradiction.
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