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Source: — Problem Solving |
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by krisraam » Wed Mar 25, 2009 5:58 pm
3 people have exactly 2 siblings. This means three of them are siblings.

4 people have exactly one sibling.

Total no of ways of selecting 2 people from 7 = 7C2 = 21

No of ways favorable ways.

1. If we select One from the group of 3(2 siblings) and one from the group of 4(1 sibling) = 3C1*4C1 = 12

2. If we select 2 from the group of 4 who are not siblings = 4C2( Total Selections) - 2 ( Selections with siblings) = 4

Probability = 16/21.

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raama
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by orel » Wed Mar 25, 2009 6:07 pm
thanks!

but i still can't understand the second step in solving a problem. can you please elaborate on that part?
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by krisraam » Wed Mar 25, 2009 6:55 pm
Feruza Matyakubova wrote:thanks!

but i still can't understand the second step in solving a problem. can you please elaborate on that part?
Four of them has exactly one sibling

a,b,c,d are the members of the group.

Assume that a,b and c,d are siblings.

2 people from 4 will be selected in 4C2 = 6 ways.

These 6 ways include selecting (a,b) and (c,d). We have to exclude them.

So 6 -2 = 4 ways we can select 2 people who are not sibllings.

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by orel » Wed Mar 25, 2009 7:12 pm
i understand now
thank you!
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by Tryingmybest » Thu Mar 26, 2009 6:32 am
No of ways favorable ways.

1. If we select One from the group of 3(2 siblings) and one from the group of 4(1 sibling) = 3C1*4C1 = 12

2. If we select 2 from the group of 4 who are not siblings = 4C2( Total Selections) - 2 ( Selections with siblings) = 4


Question: In this why are we not considering 2 from group of 3 men who are not siblings??
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by krisraam » Thu Mar 26, 2009 6:44 am
Tryingmybest wrote:No of ways favorable ways.

1. If we select One from the group of 3(2 siblings) and one from the group of 4(1 sibling) = 3C1*4C1 = 12

2. If we select 2 from the group of 4 who are not siblings = 4C2( Total Selections) - 2 ( Selections with siblings) = 4


Question: In this why are we not considering 2 from group of 3 men who are not siblings??
Each one from that group has 2 siblings.
Like A has B and C as siblings.
B has A and C as siblings.
C has A and B as siblings.

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by Tryingmybest » Thu Mar 26, 2009 7:20 am
Here is my Vision of it

A-B C- D X- Y-Z

- denotes siblings

Selecting 2 from 7 = 7 C2 = 21

Pairs which are not siblings = AC ,AD,AX,AY,AZ,BC,BD,BX,BY,BZ,CX,CY,CZ,DX,DY,DZ

So Probability = 16/21

Thanks
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