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There are six marked points on the circle above. How many different

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Joined: 02 Sep 2009
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Kudos [?]: 135467 [0], given: 12695

There are six marked points on the circle above. How many different [#permalink]

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New post 23 Nov 2017, 22:23
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There are six marked points on the circle above. How many different lines can be drawn that contain two of the marked points?

(A) 5
(B) 6
(C) 12
(D) 15
(E) 30

[Reveal] Spoiler:
Attachment:
2017-11-23_2025_003.png
2017-11-23_2025_003.png [ 4.57 KiB | Viewed 450 times ]
[Reveal] Spoiler: OA

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Kudos [?]: 135467 [0], given: 12695

Expert Post
Math Expert
User avatar
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Joined: 02 Aug 2009
Posts: 5345

Kudos [?]: 6111 [0], given: 121

Re: There are six marked points on the circle above. How many different [#permalink]

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New post 25 Nov 2017, 03:56
Bunuel wrote:
Image
There are six marked points on the circle above. How many different lines can be drawn that contain two of the marked points?

(A) 5
(B) 6
(C) 12
(D) 15
(E) 30

[Reveal] Spoiler:
Attachment:
2017-11-23_2025_003.png



The Q asks us nothing but to chose 2 out of 6 points so \(6C2 = \frac{6*5}{2}=15\)
D
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Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Kudos [?]: 6111 [0], given: 121

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Joined: 22 May 2016
Posts: 1119

Kudos [?]: 400 [1], given: 640

There are six marked points on the circle above. How many different [#permalink]

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New post 25 Nov 2017, 13:46
1
This post received
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Bunuel wrote:
Image
There are six marked points on the circle above. How many different lines can be drawn that contain two of the marked points?

(A) 5
(B) 6
(C) 12
(D) 15
(E) 30

[Reveal] Spoiler:
Attachment:
The attachment 2017-11-23_2025_003.png is no longer available

Attachment:
aaaaaa.png
aaaaaa.png [ 22.6 KiB | Viewed 130 times ]

While I agree that an approach using combinations probably is efficient for most, a very easy pattern exists here.

The pattern-finding comes in handy when under pressure and, ahem, temporarily grouchy about combinatorics.
The method was not a time-killer. 40 seconds.

Label the points to track on where you started.

Draw, and count, all possible lines from A. There will be

AB, AC, AD, AE, and AF = 5

Draw and count all possible lines from B that do not replicate the line already drawn, AB. There will be

BC, BD, BE, and BF = 4

Draw and count all possible lines from C without replication of lines already drawn. There will be

CD, CE, and CF = 3. Stop.

The pattern is clear: the number of distinct lines that can be drawn from successive points, except F, decreases by 1 for each point.

(You can't draw anything new from F. It has been a point on one of every other letter's lines. Lines drawn from F = 0.)

Number of lines that contain exactly two points = (5 + 4 + 3 + 2 + 1 + 0) = 15

Answer (D)

Kudos [?]: 400 [1], given: 640

There are six marked points on the circle above. How many different   [#permalink] 25 Nov 2017, 13:46
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