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    OlympiadMediumCombinatorics5–6

    The numbers 11 to 99 are written on nine cards. How many different pairs of cards have an odd sum?

    Os números de 11 a 99 estão escritos em nove cartões. Quantos pares diferentes de cartões têm soma ímpar?

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    Solution

    Step 1 of 3

    1. 1.A sum is odd only when one card is odd and the other is even.