A group of 25 people consists of knights, serfs and damsels. Each knight always tells the truth, each serf always lies, and each damsel alternates between telling the truth and lying. When each of them was asked: "Are you a knight?", 17 of them said "Yes". When each of them was then asked: "Are you a damsel?", 12 of them said "Yes". When each of them was then asked: "Are you a serf?", 8 of them said "Yes". How many knights are in the group?
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Grey Kangaroo 2014 (2014)
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