type in your calculator C(6,4) like the combination of 6 things taken 4 at a time
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There are: 9C6 = 84 combinations
The number of 4 different book combinations you can choose from 6 books is;6C4 =6!/[4!(6-4)!] =15 combinations of 4 different books.
The number of combinations of 6 letters is 6! or 720.
There are 33C6 = 33*32*31*30*29*28/(6*5*4*3*2*1) = 1,107,568 combinations.
(8 x 7 x 6 x 5 x 4 x 3)/(6 x 5 x 4 x 3 x 2) = 28 combinations