# 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)?

1. lajnklh

posted by Anonymous
2. 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 nPr and nCr

to do C(12,7)
enter
12
2nd F
nCr
7
=

you should get 792

posted by Reiny

First Name

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