math213
Say you have 12 books, but room to display only 7. I will open this to everyone  how many differnt arrangements are there if the order of the books matters (permutation) and how many if order is of no consequence (combination)?
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brina

lajnklh
posted by Anonymous

If the order does matter than the number of permutations is 12*11*10*9*8*7*6= P(12,7) = 3991680
if the order does not matter (combinations)
it would be 12!/(7!5!) = C(12,7) = 792
both P(n,r) and C(n,r) can be found on a standard scientific calculator
usually they are labeled _{n}P_{r} and _{n}C_{r}
to do C(12,7)
enter
12
2nd F
_{n}C_{r}
7
=
you should get 792posted by Reiny
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