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by medea66 » Wed Aug 27, 2008 1:46 pm
Is this a perm/comb?
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by anshul265 » Wed Aug 27, 2008 1:53 pm
Permutation.

nC2 = 190
n!
------- = 190
n-2! 2!

n (n-1)
-------- = 190
2

n^2 - n = 380

n^2 - n - 380 = 0

n = 20, 18.
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by pre-gmat » Wed Aug 27, 2008 3:21 pm
How did u get this part?

n (n-1)
-------- = 190
2
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by kuroneko1313 » Wed Aug 27, 2008 4:01 pm
pre-gmat wrote:How did u get this part?

n (n-1)
-------- = 190
2
Consider this:

n! = n (n-1) (n-2)!

As posted by anshul265:

n!
------- = 190
n-2! 2!

n (n-1) (n-2)!
----------------- = 190
n-2! 2!

now the (n-2)! gets canceled out by the denominator, leaving only n (n-1) at the top. Hope that helps.
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by raju232007 » Thu Aug 28, 2008 2:17 am
there are 190 possible selections to select 2 people from n members

so nC2=190

i.e n!/2!((n-2)!=190
n!=380(n-2)!

w.k.t n!=n*(n-1)*(n-2)!


n*(n-1)*(n-2)!=380(n-2)!

n(n-1)=380

Solving this we get n=20
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by vishubn » Thu Aug 28, 2008 2:49 am
i plugged in numbers

ANd got the answer with

nC2=190

where n is the number of people wanting to be filled for two slots !

20 possiblity for the first slot and 19 for the second !
as it says differnt number of ways its combi

20*19/2*1 = 10* 19=190

Vishu
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by medea66 » Fri Aug 29, 2008 5:44 am
Okay, so at the end you have n^2-n-380 and then you factor this out to get 20????
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