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# Subgroups

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VP
Joined: 06 Feb 2007
Posts: 1024
Followers: 19

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

Subgroups [#permalink]  23 Mar 2007, 11:55
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions

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
Manager
Joined: 02 Jan 2007
Posts: 211
Followers: 2

Kudos [?]: 1 [0], given: 0

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.
Manager
Joined: 12 Feb 2007
Posts: 167
Followers: 1

Kudos [?]: 1 [0], given: 0

thats smart, man ill never be able to do that
Senior Manager
Joined: 20 Feb 2007
Posts: 264
Followers: 1

Kudos [?]: 7 [0], given: 0

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?
VP
Joined: 06 Feb 2007
Posts: 1024
Followers: 19

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

The OA is E.
Good job, vikramjit_01!
Manager
Joined: 02 Jan 2007
Posts: 211
Followers: 2

Kudos [?]: 1 [0], given: 0

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.
Manager
Joined: 02 Jan 2007
Posts: 211
Followers: 2

Kudos [?]: 1 [0], given: 0

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.
Manager
Joined: 02 Jan 2007
Posts: 211
Followers: 2

Kudos [?]: 1 [0], given: 0

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.
Manager
Joined: 26 Feb 2007
Posts: 93
Followers: 1

Kudos [?]: 7 [0], given: 0

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

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