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?
a. 5/21
b. 3/7
c. 4/7
d. 5/7
e. 16/21
I don't fully understand the explanation MGMAT gave. Any help would be greatly appreciated.
MGMAT CAT PS Question 700+
This topic has expert replies
-
- Senior | Next Rank: 100 Posts
- Posts: 35
- Joined: Mon Jan 03, 2011 4:56 pm
- Location: Calif
- Thanked: 2 times
- Followed by:1 members
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?
seven people in hte room are A,B,C,D,E,F,G
4 people have exactly 1 sibling in the room => A,B,C,D ( A and B are siblings , C and D are siblings)
3 people have exactly 2 siblings in the room => E,F,G (E, F and G are siblings).
what is the probability that those two individuals are NOT siblings?
pick 1 from A,B and pick 1 from C, D (or) pick 1 from C, D and pick 1 from E,F,G (or) pick 1 from E,F,G or pick 1 from A,B
1/2*1/2 + 1/2 * 1/3+ 1/3 * 1/2 = 1/4 + 1/6 + 1/6 = 7/12.
PS: Its been more than 10 I had any touch with probability. sorry if i have miss lead you...i tried my best to answer this.
seven people in hte room are A,B,C,D,E,F,G
4 people have exactly 1 sibling in the room => A,B,C,D ( A and B are siblings , C and D are siblings)
3 people have exactly 2 siblings in the room => E,F,G (E, F and G are siblings).
what is the probability that those two individuals are NOT siblings?
pick 1 from A,B and pick 1 from C, D (or) pick 1 from C, D and pick 1 from E,F,G (or) pick 1 from E,F,G or pick 1 from A,B
1/2*1/2 + 1/2 * 1/3+ 1/3 * 1/2 = 1/4 + 1/6 + 1/6 = 7/12.
PS: Its been more than 10 I had any touch with probability. sorry if i have miss lead you...i tried my best to answer this.
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
jscpba 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?
a. 5/21
b. 3/7
c. 4/7
d. 5/7
e. 16/21
I don't fully understand the explanation MGMAT gave. Any help would be greatly appreciated.
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.
The correct answer is E.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
- anshumishra
- Legendary Member
- Posts: 543
- Joined: Tue Jun 15, 2010 7:01 pm
- Thanked: 147 times
- Followed by:3 members
nCr = n!/(r!)(n-r)!jscpba wrote:The MGMAT explanation says use:
(7*6)/2 to come to 21 combinations
Where do the 7 and 6 come from?
So, 7C2 = 7!/(2!)(7-2)! = 7!/(5!)*(2!) = (5!)*6*7/(5!)*2! (Since, 7! = 5!*6*7)
= 6*7/2 = 21.
Thanks
Anshu
(Every mistake is a lesson learned )
Anshu
(Every mistake is a lesson learned )
nice
so there is a set of 3 siblings (those who have exactly 2)
+ 2 other sets of siblings (exactly 1)
3 + 2 + 2 ---> total 7
2 of the set of 3 can be selected in 3C2 = 3
2 of 2 from second set in 1 way
2 of 2 from third set in 1 way
Total ways to select is 7C2 = 21
prob of selecting 2 siblings = 5/21
so prob of not selecting is 1 - prob of selecting 2 = 1 - 5/21 = 16/21
so there is a set of 3 siblings (those who have exactly 2)
+ 2 other sets of siblings (exactly 1)
3 + 2 + 2 ---> total 7
2 of the set of 3 can be selected in 3C2 = 3
2 of 2 from second set in 1 way
2 of 2 from third set in 1 way
Total ways to select is 7C2 = 21
prob of selecting 2 siblings = 5/21
so prob of not selecting is 1 - prob of selecting 2 = 1 - 5/21 = 16/21
-
- Master | Next Rank: 500 Posts
- Posts: 131
- Joined: Fri Jun 18, 2010 10:19 am
- Location: New York, NY
- Thanked: 10 times
man i'm having the worst time wrapping my head around probability and combinatorics! anyone have a good resource to help with this stuff? i've gone through the mgmat literature on the topics twice and still not fully getting it!
- mskgmat
- Junior | Next Rank: 30 Posts
- Posts: 24
- Joined: Mon Sep 12, 2011 8:51 am
- Thanked: 1 times
- GMAT Score:650+
Go through NOVA book. It is good.aleph777 wrote:man i'm having the worst time wrapping my head around probability and combinatorics! anyone have a good resource to help with this stuff? i've gone through the mgmat literature on the topics twice and still not fully getting it!
-
- Master | Next Rank: 500 Posts
- Posts: 218
- Joined: Wed Nov 23, 2011 8:05 pm
- Thanked: 26 times
- Followed by:4 members
This seems to be oft recommended:aleph777 wrote:man i'm having the worst time wrapping my head around probability and combinatorics! anyone have a good resource to help with this stuff? i've gone through the mgmat literature on the topics twice and still not fully getting it!
https://www.amazon.com/gp/product/193624 ... PDKIKX0DER
Good luck!