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Subgroups [#permalink] New post 23 Mar 2007, 11:55
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
Please provide explanations.

From a group of 8 people, subgroups of 4 people are formed. Andy and Kelly are in the eight-people group. If a sub-group is picked at random, what is the probability of picking a sub-group that includes Andy but not Kelly?

A)1/7
B)1/4
C)1/3
D)1/2
E)2/7
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 [#permalink] New post 23 Mar 2007, 12:14
Probablitity = Favorable cases / Total cases

Total cases = 8C4 = 70

Favorable = 6C3 (6 and not 8 because we are including Andy and excluding Kelly so we are left with 6 people to choose from and we need to select 3 now since 1 has already been selected.)

6C3 = 20


P = 20/70 = 2/7

So its E for me.
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 [#permalink] New post 23 Mar 2007, 12:50
thats smart, man ill never be able to do that
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 [#permalink] New post 24 Mar 2007, 00:28
I am not good at this kind of sums :(

What if the question is:

From a group of 8 people, subgroups of 4 people are formed. Andy is in the eight-people group. If a sub-group is picked at random, what is the probability of picking a sub-group that includes Andy? OR how many combinations will be formed including Andy?
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 [#permalink] New post 24 Mar 2007, 12:30
The OA is E.
Good job, vikramjit_01!
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 [#permalink] New post 26 Mar 2007, 07:52
Summer3 wrote:
What if the question is:

From a group of 8 people, subgroups of 4 people are formed. Andy is in the eight-people group. If a sub-group is picked at random, what is the probability of picking a sub-group that includes Andy?


1/2 for me. Andy would be either in group 1 or group 2.
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 [#permalink] New post 26 Mar 2007, 07:58
Summer3 wrote:
how many combinations will be formed including Andy?


I'm assuming this means if a subgroup of 4 from a total of 8 people is formed, how many ways can we form a subgroup that includes Andy.

Here are the 4 spots to be filled in the subgroup.
_ _ _ _

Lets give the first one to Andy because we want him to be in the subgroup, then we now have
A _ _ _

The remaining 3 spots can be filled in
7*6*5 ways
so 210 subgroups if I understood your question correctly.
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 [#permalink] New post 26 Mar 2007, 08:24
Tuneman wrote:
thats smart, man ill never be able to do that


Summer3 wrote:
I am not good at this kind of sums :(


Try this developmental plan:

Day 1:
Take about 25 questions that are half probability and the other half Perms & Combs of no less than Medium difficulty.
Do them all. Doesn't matter what number you get right or wrong. Once done understand the way the problems are meant to be solved.

Day 2:
Do them all again the next day. Also, goto gmatclub.com > Courses > Quantitive and understand the tutorials there.

Day 3:
Now you will be all set to surprise yourself, except for one more thing -
Do about 4-5 questions from the file at this link everyday. Its important to keep in touch.
http://www.gmatclub.com/phpbb/viewtopic.php?t=19417

Take out about an hours or 2 hours time every week (weekend's work for me) to revise the stuff you did on Days 1 & 2.

If you've been crazy enough to do the above, my guess is you'll pray for Perms & Combs, and Probability to come in your GMAT.
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 [#permalink] New post 26 Mar 2007, 13:57
From a group of 8 people, subgroups of 4 people are formed. Andy and Kelly are in the eight-people group. If a sub-group is picked at random, what is the probability of picking a sub-group that includes Andy but not Kelly?

A)1/7
B)1/4
C)1/3
D)1/2
E)2/7

possible no. of groups =8C4=70
possible groups with andy without kelly =6C3=20

so probability of such a group =2/7

Please post the OA
  [#permalink] 26 Mar 2007, 13:57
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