From a group of 5 girls and 4 boys, 7 are to be randomly selected. what is the probability that 4 girls will be selected.
A) 4/9 B) 3/5 C) 5/9 D) NONE OF THESE
Probability problem
This topic has expert replies
- MubbashirAbbas
- Junior | Next Rank: 30 Posts
- Posts: 12
- Joined: Wed May 23, 2012 10:01 am
GMAT/MBA Expert
- Anurag@Gurome
- GMAT Instructor
- Posts: 3835
- Joined: Fri Apr 02, 2010 10:00 pm
- Location: Milpitas, CA
- Thanked: 1854 times
- Followed by:523 members
- GMAT Score:770
Total number of ways to select 7 persons from 5 girls and 4 boys = 9C7 = 9*8/2 = 36MubbashirAbbas wrote:From a group of 5 girls and 4 boys, 7 are to be randomly selected. what is the probability that 4 girls will be selected.
Number of ways to select 4 girls from 5 girls = 5C4 = 5
Number of ways to select (7 - 4) = 3 boys from 4 boys = 4C3 = 4
Hence, number of ways to select 7 persons from a group of 5 girls and 4 boyssuch that 4 girls will be selected = 5*4 = 20
Hence, the required probability = 20/36 = 5/9
The correct answer is C.
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
- MubbashirAbbas
- Junior | Next Rank: 30 Posts
- Posts: 12
- Joined: Wed May 23, 2012 10:01 am
GMAT/MBA Expert
- Anurag@Gurome
- GMAT Instructor
- Posts: 3835
- Joined: Fri Apr 02, 2010 10:00 pm
- Location: Milpitas, CA
- Thanked: 1854 times
- Followed by:523 members
- GMAT Score:770
Because 36 is the total number of possible selections without any restriction.MubbashirAbbas wrote:Why did u put 36 in the denominator in the last step
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
-
- Newbie | Next Rank: 10 Posts
- Posts: 2
- Joined: Wed Mar 07, 2012 4:30 am
(5c4*4c3)/9c7=(5*4)/36=5/9
since the total no of girls are 5 and the group should consist of 4 girls,so 4 girls can be selected in 5c4 way and we need 3 more people to form the group of 7(7-4=3) hence they can be selected from boys in 4c3 manner.this forms our favorable condition i,e 5c4*4c3
the 9c7 denotes all possible casses of forming the group irrespective of boys n girls and hence forming the total possible conditions.
since the total no of girls are 5 and the group should consist of 4 girls,so 4 girls can be selected in 5c4 way and we need 3 more people to form the group of 7(7-4=3) hence they can be selected from boys in 4c3 manner.this forms our favorable condition i,e 5c4*4c3
the 9c7 denotes all possible casses of forming the group irrespective of boys n girls and hence forming the total possible conditions.
- Stuart@KaplanGMAT
- GMAT Instructor
- Posts: 3225
- Joined: Tue Jan 08, 2008 2:40 pm
- Location: Toronto
- Thanked: 1710 times
- Followed by:614 members
- GMAT Score:800
Just to elaborate a bit more, here's the probability formula:MubbashirAbbas wrote:Why did u put 36 in the denominator in the last step
probability = (# of desired outcomes)/(total # of possibilities)
and, as Anurag noted, 36 was the total number of possible choices.
Whenever you're doing a probability problem, starting out by writing that equation on your scratch paper will help you see exactly what you need to calculate.
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
- MubbashirAbbas
- Junior | Next Rank: 30 Posts
- Posts: 12
- Joined: Wed May 23, 2012 10:01 am
You guys used combination formula. Could u specify when should I use permutation and when to use combination. Something like a rule of thumb. I understand that permutation is used when order is important. Thank u all for helping me out.
- Stuart@KaplanGMAT
- GMAT Instructor
- Posts: 3225
- Joined: Tue Jan 08, 2008 2:40 pm
- Location: Toronto
- Thanked: 1710 times
- Followed by:614 members
- GMAT Score:800
That's the exact rule!MubbashirAbbas wrote: I understand that permutation is used when order is important.
If you just care about which objects to select, then you use the combinations formula.
If you care not only about which objects to select, but also the order of selection (or arrangement), then you use the permutations formula.
Here are two examples to illustrate the difference:
1) 7 students are trying out for the school tennis team. If there are 4 spots on the team, how many different teams can be made out of the 7 students?
In this question, we only care about WHICH students we select; in other words, we want to know how many teams of 4 we can make out of the 7 applicants. Since order is irrelevant, we use COMBINATIONS.
2) 7 goats are entered in the county fair. There are ribbons awarded for first, second and third place. How many different arrangements of winners can be made out of the 7 goats?
In this question, we care not only about WHICH goats get selected but ALSO about the ORDER in which they're selected. Since we care about order, we use PERMUTATIONS.
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
- aneesh.kg
- Master | Next Rank: 500 Posts
- Posts: 385
- Joined: Mon Apr 16, 2012 8:40 am
- Location: Pune, India
- Thanked: 186 times
- Followed by:29 members
Thumb Rules of 'Permutations & Combinations':MubbashirAbbas wrote: Could u specify when should I use permutation and when to use combination. Something like a rule of thumb. I understand that permutation is used when order is important. Thank u all for helping me out.
(i) OR is Addition, AND is addition.
(ii)The number of ways of selecting or choosing or picking 'r' objects from 'n' different objects is given by nCr. The 'C' stands for 'Combinations'.
Key words for using Combinations: selecting, choosing, picking (and other tense forms of these words).
For example:
In how many ways can a 2-member team be chosen from a group of 4 people: A, B, C and D?
4C2
(iii)The number of ways of arranging or distributing 'n' different objects on 'r' places (where r < n) is given by nPr. The 'P' stands for 'Permutations'.
Key words for using Permutations: arranging, distributing(and other tense forms of these words).
For example:
In how many ways can three prizes be distributed among the three winners in a contest in which five people participated?
5P3 or 5C3*3!
(iv) Number of ways of arranging/distributing 'n' different objects on 'n' places is given by nPn or n!
P.S.: More than the rules listed above, it is important that each of the above concepts is well-understood.
Aneesh Bangia
GMAT Math Coach
[email protected]
GMATPad:
Facebook Page: https://www.facebook.com/GMATPad
GMAT Math Coach
[email protected]
GMATPad:
Facebook Page: https://www.facebook.com/GMATPad
- 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
For an alternate approach to this sort of problem, check my posts here:
https://www.beatthegmat.com/select-exact ... 88786.html
https://www.beatthegmat.com/rain-prob-t88396.html
https://www.beatthegmat.com/johnny-the-g ... 85956.html
https://www.beatthegmat.com/select-exact ... 88786.html
https://www.beatthegmat.com/rain-prob-t88396.html
https://www.beatthegmat.com/johnny-the-g ... 85956.html
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