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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八本书挑出四本书的总共可能性（C48）=70种，减去四本书都从hardcover里出来的可能性（C46）（15可能性）=70-15=55种可能性
c84=8!/4!，而不是(8*7*6*4)/2
我去～～我竟然看成了求概率，题目就是要求几种可能性，没有要求概率！！！
逆向思维
It's almost always better to solve "at least one" combinations/probability questions with {at least one}={total}-{none}. Total # of selections of 4 books out of 8: C48=70C84=70; Total # of selections of 4 hardback books out of 6 (so none paperback): C46=15C64=15; {at least one} = {total} - {none} = 70 - 15 = 55.
at least!!!! 做题不能贪快啊
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八本书挑出四本书的总共可能性（C48）=70种，减去四本书都从hardcover里出来的可能性（C46）（15可能性）=70-15=55种可能性
c84=8!/4!，而不是(8*7*6*4)/2
我去～～我竟然看成了求概率，题目就是要求几种可能性，没有要求概率！！！
逆向思维
It's almost always better to solve "at least one" combinations/probability questions with {at least one}={total}-{none}.
Total # of selections of 4 books out of 8: C48=70C84=70;
Total # of selections of 4 hardback books out of 6 (so none paperback): C46=15C64=15;
{at least one} = {total} - {none} = 70 - 15 = 55.
at least!!!! 做题不能贪快啊