document.write( "Question 284028: How many ways are there to select 3 books from a shelf of 20 books? \n" ); document.write( "

Algebra.Com's Answer #206131 by solver91311(24713) $\"\"$ $\"About$

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\n" ); document.write( "\n" ); document.write( "It actually depends on whether or not the order in which you select the books matters. One must presume that the shelf contains 20 different books as well because that fact is a factor in answer to this question.\r
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\n" ); document.write( "\n" ); document.write( "If you don't care about the order, you need the number of combinations of 20 things taken 3 at a time:\r
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\n" ); document.write( "\n" ); document.write( "On the other hand, if the subset War and Peace, Differential Equations for Dummies, and The Cat in the Hat, is different from the subset The Cat in the Hat, War and Peace, and Differential Equations for Dummies, then you need the number of permutations of 20 things taken 3 at a time:\r
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\n" ); document.write( "\n" ); document.write( "Notice that the only difference is the 3! that is in the denominator of the combination calculation but not in the permutation calculation.\r
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\n" ); document.write( "\n" ); document.write( "You can do your own arithmetic.\r
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