There are 8 books on a shelf, of which 2 are paperbacks and 6 are hardbacks. How many possible selections of 4 books from this self include at least one paperback?
40
45
50
55
60
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c84=8!/4!，而不是(8*7*6*4)/2
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c84=8!/4!，而不是(8*7*6*4)/2
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