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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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.
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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.
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