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题目:
on a bookshelf,there are five different mystery books and six different biographies,How many
different set of four books can Emilio choose if two of the books must be mystery books and
two of the books must be biographies??
这一类型的题目我一点都不会,什么排列,什么组合的,P,C,
这一个内容是在中学的哪个阶段教的啊?国外的书里有涉及但我想知道中国的教科书是否有相关内容
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