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# Alan has a flock of sheep from which he will choose 4 to

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SVP
Joined: 16 Oct 2003
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Alan has a flock of sheep from which he will choose 4 to [#permalink]  11 Dec 2005, 19:07
15. Alan has a flock of sheep from which he will choose 4 to take with him to the livestock show in Houston. If Alan has 15 distinct possible groups of sheep he could take to the show then which of the following is the number of sheep in his flock?
(A) 30
(B) 15
(C) 7
(D) 6
(E) 5
Director
Joined: 09 Oct 2005
Posts: 722
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Re: Count Sheep [#permalink]  11 Dec 2005, 19:22
Bhai wrote:
15. Alan has a flock of sheep from which he will choose 4 to take with him to the livestock show in Houston. If Alan has 15 distinct possible groups of sheep he could take to the show then which of the following is the number of sheep in his flock?
(A) 30
(B) 15
(C) 7
(D) 6
(E) 5

My vote for D
4 sheeps from X 15 possblie ways So X=6 Because Only this way you can get 15 possible arrangments while choosing 4 sheeps!
Here is formula C4,X=15 lets substitute x with answer choices!In "which of the following"questions I start with the D or E
When X= 6 C4,6=6*5*4*3/1*2*3*4=15 Bingo
GMAT Club Legend
Joined: 07 Jul 2004
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Location: Singapore
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X is the number of sheeps in the flock. Distinct groups, so use combination

XC4 = 15

Solve by substitution:

30C4 = too big
15C4 = too big
6C4 = 15. (Correct)
SVP
Joined: 16 Oct 2003
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You guys rule.
Current Student
Joined: 29 Jan 2005
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I also backsolved it to get 6----> 6C4= 15. Is there a quicker method?
VP
Joined: 22 Aug 2005
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GMATT73 wrote:
I also backsolved it to get 6----> 6C4= 15. Is there a quicker method?

backsolving is most efficient in this problem, IMO.
_________________

Whether you think you can or think you can't. You're right! - Henry Ford (1863 - 1947)

Director
Joined: 26 Sep 2005
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Location: Munich,Germany
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thats right. backsolve-d
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