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
登录 或 注册 后可以参加讨论
扫描二维码，直接登录
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.