Ten players play tennis. If every pair of players plays exactly one match against each other, how many matches are played?

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• A 21
• B 28
• C 36
• D 45

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Answer: D. 45

Call the players A to J. A plays 9 matches, B plays 8 new ones, C plays 7, and so on: 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 45 matches.

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