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two elements

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by theCEO » Fri Jul 06, 2012 12:46 am
number of 2 element subsets = (4*3)/2 = 6

number of (2 and 4 elements) = 1 [only 1 arrangement]

therefore no of pairs that do not contain the pair of 2 and 4 = 6-1=5
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by coolhabhi » Fri Jul 06, 2012 4:54 am
IMO two.

take a list of all the two element subsets of (1,2,3,4). We have
(1,2) (1,3) (1,4)
(2,1) (2,3) (2,4)
(3,1) (3,2) (3,4)
(4,1) (4,2) (4,3)

An other way to look at it is :
If two elements are removed then the set will be (1,3).
So we can get on 2! subsets. that is 2.

BTW what is the OA?
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by Brent@GMATPrepNow » Fri Jul 06, 2012 5:48 am
nafiul9090 wrote:how many two element subsets of (1,2,3,4) are there that do not contain the pair of elements 2 and 4

one
two
three
four
five
Given the small numbers in the answer choices, another viable approach is to simply list the allowable outcomes:
(1,2), (1,3), (1,4), (2,3), (3,4)

So, the answer is E - five

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by theCEO » Fri Jul 06, 2012 3:07 pm
coolhabhi wrote:IMO two.

take a list of all the two element subsets of (1,2,3,4). We have
(1,2) (1,3) (1,4)
(2,1) (2,3) (2,4)
(3,1) (3,2) (3,4)
(4,1) (4,2) (4,3)

An other way to look at it is :
If two elements are removed then the set will be (1,3).
So we can get on 2! subsets. that is 2.

BTW what is the OA?
I think you are answering the question - "How many 2-sets that do not contain either 2 or 4?"
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