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# If one of two parallel lines has 7 points on it and the

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Manager
Joined: 05 Aug 2008
Posts: 88
Schools: McCombs Class of 2012
If one of two parallel lines has 7 points on it and the  [#permalink]

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03 Nov 2008, 10:56
If one of two parallel lines has 7 points on it and the other has 8, how many triangles can be drawn using the points?

a) 168
b) 196
c) 316
d) 364
e) 455

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VP
Joined: 17 Jun 2008
Posts: 1476

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03 Nov 2008, 11:02
1
D. 364.

Total number of triangles = 7C2*8 + 8C2*7
Manager
Joined: 05 Aug 2008
Posts: 88
Schools: McCombs Class of 2012

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03 Nov 2008, 11:04
Thanks for the reply scthakur. Can you explain the logic you used to arrive at that? Thank you.
Current Student
Joined: 28 Dec 2004
Posts: 3287
Location: New York City
Schools: Wharton'11 HBS'12

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03 Nov 2008, 11:10
1
basically here is what you are doing

triangle has 3 points..right..

so say on 7 point line you pick any 2 points then on the 8 point line you have to pick 1 point..

7C2*8<--so basically the 2 points are on the 7 point line and the 1 point is on the 8 point line and you have 8 possibilities of picking that 1 point.

next the 2 points of a triangle are on the 8point line so 8C2*7 <-- since now you have 1 pont on the 7point line and as such you have 7 possibilities..
VP
Joined: 17 Jun 2008
Posts: 1476

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03 Nov 2008, 11:13
smarinov wrote:
Thanks for the reply scthakur. Can you explain the logic you used to arrive at that? Thank you.

Good explanation from freshinha is there now.
Manager
Joined: 05 Aug 2008
Posts: 88
Schools: McCombs Class of 2012

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03 Nov 2008, 11:15
Thank you both. I get it now.

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: Number of Triangles &nbs [#permalink] 03 Nov 2008, 11:15
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# If one of two parallel lines has 7 points on it and the

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