There are 10 books on a shelf, of which 4 are paperbacks and 6 are hardbacks. How many possible selections of 5 books from the shelf contain at least one paperback and at least one hardback?
75
120
210
246
252
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没有一本paperback或者没有一本hardback的可能性是C5/6
（事实上取出5本书不可能只取到paperback，因为paperback只有4本，即不用考虑C5/4
所以只要考虑全部取到hardback（即没有paperback）的情况）
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