Boy or Girl?
Three siblings were locked up in a room by an evil spirit. He cast a spell on all of them such that they forgot about their own gender in an instant! However, they could still recognise the gender of their two siblings.
The evil spirit said. "If all of you can correctly identify your gender within thirty minutes, I will release all of you. However, if any one of you communicate with each other in any form or identify your own gender wrongly, I will execute all of you! To help you, I give you a hint: at least one of you is a boy."
One of the siblings saw his/her brother and sister in front of him/her. He/She waited for fifteen minutes and still heard no response from any of them. Is this sibling a brother or a sister?
Details and assumptions:
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The evil spirit did not lie.
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All three siblings still have their thinking faculties intact.
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Wanting to be free as soon as possible, they try to think fast and strategically.
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The brother identified in the question saw his sister and his other sibling (of gender X) where X is unknown.
Since he could not identify his gender after 15 minutes, he must be unable to arrive at a conclusion about his gender after seeing his sister and sibling X. If sibling X was a girl, then he can immediately tell that he must be a boy as the evil spirit hinted that at least one is a boy.
Hence, sibling X must be a b r o t h e r .