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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C10.5-C6.5. 不是C10.5/C6.5 ,减去里面有全选同一种的情况 ....................................
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C10.5-C6.5. 不是C10.5/C6.5 ,减去里面有全选同一种的情况
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