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Siblings

by neerajkumar1_1 » Mon Sep 13, 2010 3:53 am
In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?
5/21
3/7
4/7
5/7
16/21

This question has been answered before on the forum...
but I cant understand the concept of siblings here...

4 people have exactly 1 sibling means what... and 3 people have exactly 2 siblings means what...

Plz explain... thanks...
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by Jim@Grockit » Mon Sep 13, 2010 5:16 am
neerajkumar1_1 wrote:In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?
5/21
3/7
4/7
5/7
16/21

This question has been answered before on the forum...
but I cant understand the concept of siblings here...

4 people have exactly 1 sibling means what... and 3 people have exactly 2 siblings means what...

Plz explain... thanks...
"Siblings" means "brothers or sisters;" for simplicity's sake, consider replacing "sibling" in your head with "relative." It means that 4 people have one relative in the room, and 3 people have two relatives in the room. Does that help?
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by neerajkumar1_1 » Mon Sep 13, 2010 5:23 am
Jim@Grockit wrote:
neerajkumar1_1 wrote:In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?
5/21
3/7
4/7
5/7
16/21

This question has been answered before on the forum...
but I cant understand the concept of siblings here...

4 people have exactly 1 sibling means what... and 3 people have exactly 2 siblings means what...

Plz explain... thanks...
"Siblings" means "brothers or sisters;" for simplicity's sake, consider replacing "sibling" in your head with "relative." It means that 4 people have one relative in the room, and 3 people have two relatives in the room. Does that help?
Well thats not the issue..
I saw the solns...
How does one sibling for 4 people relate to 2 pairs and so on for 3 people with 2 siblings...
I am just getting thoroughly confused and am not able to visualize the problem...

Hope u can provide me with a little more detail...
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by neerajkumar1_1 » Tue Sep 14, 2010 2:54 am
hi guyz... can someone plz provide me with a solution and including how we make the sets of siblings...
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by kmittal82 » Tue Sep 14, 2010 3:10 am
>people have exactly 1 sibling

Let them be A1 A2 B1 B2

Siblings: A1-A2 and B1-B2

>3 people have exactly 2 siblings in the room

C1 C2 C3

Siblings: C1-C2-C3

To further simplify, you can say the people are A A B B C C C, but keep in mind when doing any selections, the order will matter (so use a permutation)
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by neerajkumar1_1 » Tue Sep 14, 2010 3:14 am
kmittal82 wrote:>people have exactly 1 sibling

Let them be A1 A2 B1 B2

Siblings: A1-A2 and B1-B2

>3 people have exactly 2 siblings in the room

C1 C2 C3

Siblings: C1-C2-C3

To further simplify, you can say the people are A A B B C C C, but keep in mind when doing any selections, the order will matter (so use a permutation)
hi,
In the fist set which u made..
where it is given that 4 people have one sibling...

how do u make 2 relations
Siblings: A1-A2 and B1-B2

I am just failing to interpret the question.. :(
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by GMATGuruNY » Tue Sep 14, 2010 3:51 am
neerajkumar1_1 wrote:In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?
5/21
3/7
4/7
5/7
16/21

This question has been answered before on the forum...
but I cant understand the concept of siblings here...

4 people have exactly 1 sibling means what... and 3 people have exactly 2 siblings means what...

Plz explain... thanks...
Let's say that the 7 people are ABCDEFG.

4 people have exactly 1 sibling:
Let's say that A and B are siblings and that C and D are siblings.
This means:
A has 1 sibling (B).
B has 1 sibling (A).
C has 1 sibling (D).
D has 1 sibling (C).

3 people have exactly 2 siblings:
Let's say that E, F and G are all siblings of each other.
This means:
E has 2 siblings (F and G).
F has 2 siblings (E and G).
G has 2 siblings (E and F).

Total number of sibling pairs = 5: AB, CD, EF, EG, FG.
Total number of pairs that can be formed from 7 people: 7C2 = 21.
P(sibling pair) = 5/21
P(not sibling pair) = 1 - 5/21 = 16/21.

Does this help?
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by neerajkumar1_1 » Tue Sep 14, 2010 4:09 am
GMATGuruNY wrote:
neerajkumar1_1 wrote:In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?
5/21
3/7
4/7
5/7
16/21

This question has been answered before on the forum...
but I cant understand the concept of siblings here...

4 people have exactly 1 sibling means what... and 3 people have exactly 2 siblings means what...

Plz explain... thanks...
Let's say that the 7 people are ABCDEFG.

4 people have exactly 1 sibling:
Let's say that A and B are siblings and that C and D are siblings.
This means:
A has 1 sibling (B).
B has 1 sibling (A).
C has 1 sibling (D).
D has 1 sibling (C).

3 people have exactly 2 siblings:
Let's say that E, F and G are all siblings of each other.
This means:
E has 2 siblings (F and G).
F has 2 siblings (E and G).
G has 2 siblings (E and F).

Total number of sibling pairs = 5: AB, CD, EF, EG, FG.
Total number of pairs that can be formed from 7 people: 7C2 = 21.
P(sibling pair) = 5/21
P(not sibling pair) = 1 - 5/21 = 16/21.

Does this help?
it surely does... phew!!!!>.. I was like so frustrated...
thanks a lot...
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