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Re: Permutations 1 [#permalink]
23 Aug 2009, 20:09

rohansherry wrote:

IMO B. 8c3 - 1

A is the general stmt. we know that Triangle is fromed using only 3 points.

Agree with B. But I think the combination formula might be wrong.

IMHO, only 3 points are collinear, all the other 5 points are not on the same line. Hence, each of the 3 points can be combined with 2 of the 5 points to create a triangle.

Therefore, the # of triangles = 3 x 5C2 = 3 x 10 = 30.

Last edited by Jivana on 23 Aug 2009, 20:22, edited 1 time in total.

Re: Permutations 1 [#permalink]
23 Aug 2009, 20:20

Jivana wrote:

rohansherry wrote:

IMO B. 8c3 - 1

A is the general stmt. we know that Triangle is fromed using only 3 points.

Agree with B. But I think the combination formula might be wrong.

IMHO, only 3 points are collinear, all the other 5 points are not on the same line. Hence, each of the 3 points can be combined with 2 of the 5 points to create a triangle.

Therefore, the # of triangles = 3 x 5C3 = 3 x 10 = 30.

You forgot 2 of the 3 points (collinear) and 1 of the 5 points to form a triangle.
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Re: Permutations 1 [#permalink]
25 Aug 2009, 05:35

HI Guys, My answer would also be B. But, I am not clear in the possibilities to form triangles. 1. using the 3 non collinear : 5C3 2. using one collinear and 2 non collinear : 3C1x5C2

I am sure that I am missing something here. Would you please be of any help to understand the 55 possibilities?

Re: Permutations 1 [#permalink]
26 Aug 2009, 04:27

Guys, I am sorry that it took me so long to post the OA. But here it is:

1. is insufficient because it just states a well known fact 2. is sufficient because in such case we can calculate the number of triangles that can be formed: 5C3+8*3C2+3C1*5C2

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