Q30: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?
A. 40
B. 45
C. 50
D. 55
E. 60
答案C,我算出来D, C(4,8)-C(4,6)=55
or C(1,2)*C(3,6)+C(4,6)=55
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