Subgroups

Question

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

vikramjit_01 · Answer

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.

Tuneman · Answer

thats smart, man ill never be able to do that

Summer3 · Answer

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?

nervousgmat · Answer

The OA is E. 
Good job, vikramjit_01!

vikramjit_01 · Answer

1/2 for me. Andy would be either in group 1 or group 2.

vikramjit_01 · Answer

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.

vikramjit_01 · Answer

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. https://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.

kumarajeet06 · Answer

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

Subgroups

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards