公布答案：值一万美金的小智力题 结果也许让你吃惊！

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Anyone who thinks in the first way must have never learn any statistics. The sex of the first one has nothing to do with the sex of the second one. www.ddhw.com

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or not does not matter. The probability of the sex of the second is the same. The second analysis in your argument is the only right analysis. I don't understand how do you get the first analysis. Your first analysis is just wrong. The following problems have the same answer: 1. I have two kids, the first is a boy, what is the probability of the second one is a boy? 2. I have two kids, what is the probability of the second one is a boy?   They both 50%.   www.ddhw.com

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And for the first analysis, you missed the girl, girl instance, then you had 1/3 result. Whether he tell you one of the kid is boy or not can not eliminate the girl, girl instance. That is the common mistake when people start learning probability. www.ddhw.com

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I just want to use "stupid". Go ask a statistics professor in a college. The two questions looked different, but they have the same answer.www.ddhw.com A common mistake for students who learn probabilty theory: Instantiate instances based on the condition given, then calculate. Rather, you should instantiate all instances first, then determine the rate by conditions.   Therefore, your first question, you should have 4 instances rather than 3, so that it should be 50% rather thahn 1/3, don't be fooled by the condition "one of then is a boy".   Anyway, I don't know why I bother tell you this. Please just go to ask a stat professor.   www.ddhw.com