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# If there are four distinct pairs of brothers and sisters, then in how

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Director
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If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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28 Aug 2010, 16:14
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If there are four distinct pairs of brothers and sisters, then in how many ways can a committee of 3 be formed and NOT have siblings in it?

A. 8
B. 24
C. 32
D. 56
E. 192

M02-05
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Posts: 62498
Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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06 Sep 2010, 12:10
20
21
SnehaC wrote:
The first person on the committee can be anyone of the 8.

The second person on the committee can be only one out of 6 (the first person with her or his sibling excluded).

The third person can be selected only out of 4 (first two members and their siblings excluded), giving 8*6*4 permutations. These are called permutations because we count every distinct combination of 3 people 6 times (6=3!). For example, we count {Ann, Bill, and Jane} as {Ann, Bill, Jane}, {Ann, Jane, Bill}, {Jane, Ann, Bill}, {Jane, Bill, Ann}, {Bill, Ann, Jane}, {Bill, Jane, Ann}. To get the number of combinations, divide the number of permutations by 6: 8*6*4 / (6) = 32 combinations.

I understand uptil the last part but I don't understand why we're diving by 6? Can anyone clarify this for me?

We need to divide $$8*6*4=192$$ by the factorial of the # of people - 3! to get rid of duplications $$8*6*4=192$$ contains ---> $$\frac{192}{3!}=32$$ - correct answer.

Consider example with smaller number: there are two couples and we want to choose 2 people not married to each other.
Couples: $$A_1$$, $$A_2$$ and $$B_1$$, $$B_2$$. Committees possible:

$$A_1,B_1$$;
$$A_1,B_2$$;
$$A_2,B_1$$;
$$A_2,B_2$$.

Only 4 such committees are possible.

If we do as proposed in the solution you posted:
The first person on the committee can be anyone of the 4.
The second person on the committee can be only one out of 2 (the first person with her or his sibling excluded).

So we'll get: 4*2=8, so more than 4, which means that 8 contains some duplications. And to get the right answer we should divide 8 by 2! --> 8/2!=4.

It's because if you pick A1 for the first pick and than pick B1 for the second you'll get the committee {A1, B1} but if you'll pick B1 for the first pick and then A1 you'll get the exact same committee {A1, B1} (dividing by the factorial of the # of people in committee you'll exclude this double countings).

Hope it helps.
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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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31 Aug 2010, 23:28
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let AABBCCDD is the group with same letter representing a sibling pair.

total # ways to select 3 from 8 is 8C3=56

Qtn: committee of 3 NOT having siblings in it = Total (56) - committee of 3 with siblings in it

committee of 3 with siblings = select 2As and one from rem. 6. This can be done in 4 ways as 4 different letters A,B,C and D
= 4 * (2C2*6C1) = 24

Hence Answer = 56-24 = 32
##### General Discussion
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Posts: 62498
Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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28 Aug 2010, 16:32
13
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seekmba wrote:
If there are four distinct pairs of brothers and sisters, then in how many ways can a committee of 3 be formed and NOT have siblings in it?

 8
 24
 32
 56
 192

I find it difficult to understand the difference between permutation and combination and hence find these questions very hard.

As committee shouldn't have siblings in it, then a pair can send only one "representative" to the committee. # of ways to choose which 3 pairs of brothers and sisters should send one "representative" to the committee is $$C^3_4$$ (choosing 3 pairs which will be granted the right to send one "representative" to the committee);

But each of these 3 pairs can send 2 persons to the committee either a brother or a sister: $$2*2*2=2^3$$;

So total # of ways is $$C^3_4*2^3=32$$.

Similar problems:
confuseddd-99055.html?hilit=married
ps-combinations-94068.html?hilit=married
combination-permutation-problem-couples-98533.html?hilit=married

Hope it helps.
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Posts: 41
Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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06 Sep 2010, 11:29
2
2
The first person on the committee can be anyone of the 8.

The second person on the committee can be only one out of 6 (the first person with her or his sibling excluded).

The third person can be selected only out of 4 (first two members and their siblings excluded), giving 8*6*4 permutations. These are called permutations because we count every distinct combination of 3 people 6 times (6=3!). For example, we count {Ann, Bill, and Jane} as {Ann, Bill, Jane}, {Ann, Jane, Bill}, {Jane, Ann, Bill}, {Jane, Bill, Ann}, {Bill, Ann, Jane}, {Bill, Jane, Ann}. To get the number of combinations, divide the number of permutations by 6: 8*6*4 / (6) = 32 combinations.

I understand uptil the last part but I don't understand why we're diving by 6? Can anyone clarify this for me?
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Joined: 10 Aug 2009
Posts: 2
Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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06 Sep 2010, 17:54
Bunuel, i like the way u use quick formulas for permutations and combinations..can u let me know how can i get these formulas? i want to strengthen my skills on this subject..

Thanks!
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Posts: 62498
Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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07 Sep 2010, 03:11
1
vdixit wrote:
Bunuel, i like the way u use quick formulas for permutations and combinations..can u let me know how can i get these formulas? i want to strengthen my skills on this subject..

Thanks!

Probability and Combinatorics chapters of Math Book:

math-probability-87244.html
math-combinatorics-87345.html

You can also see Probability and Combinatorics questions to practice at: viewforumtags.php

Hope it helps.
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Re: PS - Combinatorics (m02q05)  [#permalink]

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04 Oct 2011, 19:56
Bunuel wrote:
The third way is also valid, but you should divide 8C1x6C1x4C1 by 3! to get rid of duplications.

With 8C1x6C1x4C1 you can have ABC members as well as BCA members, which is basically the same group.

Bunnel, how you came to the conclusion that 8C1x6C1x4C1 should be divided by 3! to get rid of duplications. Why not 4! or 2!. Please explain.
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18 Jan 2012, 03:30
1
GMATmission wrote:
Bunnel, how you came to the conclusion that 8C1x6C1x4C1 should be divided by 3! to get rid of duplications. Why not 4! or 2!. Please explain.

We divide by the number of members in the committee, so by 3!.

Consider this: 8C1*6C1*4C1 will give you all committees of ABC possible - (ABC), (ACB), (BAC), (BCA), (CAB) and (CBS) which are the SAME committee of 3 (3 distinct letters can be arranged in 3! ways). So we should divided 8C1*6C1*4C1 by 3!.

This question and the same doubt you have is also discussed here: http://gmatclub.com/forum/if-there-are- ... 99992.html

Similar questions to practice:
http://gmatclub.com/forum/ps-combinations-94068.html
http://gmatclub.com/forum/ps-combinations-101784.html
http://gmatclub.com/forum/committee-of-88772.html
http://gmatclub.com/forum/if-4-people-a ... 99055.html
http://gmatclub.com/forum/combination-p ... 98533.html

Hope it helps.
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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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27 Dec 2012, 18:38
2
1
How many ways to select 3 of the pairs with representative in the group from 4 pairs? 4!/3!1! = 4
How many ways to select a representative from each pair? 2 x 2 x 2 = 8
$$4*8 = 32$$

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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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25 Jan 2013, 00:26
2
2
shikhar wrote:
There are four distinct pairs of brothers and sisters. In how many ways can a committee of 3 be formed and NOT have siblings in it?

(A) 8
(B) 24
(C) 32
(D) 56
(E) 80

I got 32 with 4c3 * 2*2*2.
But why is 8c1 * 6c1 * 4c1 wrong ??/

You need to divide $$C^8_1*C^6_1*C^4_1$$with $$3!$$ to eliminate the duplicates (as the order in the arrangement does not matter).

Number of ways a committee of 3 be formed and NOT have siblings in it = $$\frac{C^8_1*C^6_1*C^4_1}{3!} = \frac{8*6*4}{6}=32$$

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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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11 Jul 2014, 22:42
1
we can select 8 people for 1st place, 6 for second (only one from pair can be selected) and , 4 for 3rd
So we can have total = 8*6*4 = 192
Now in above calculation, we have counted all no of ways. (ABC is different from ABE) so we have to divide the above value with no of ways we can select 3 people

No. of ways to select 3 people = 3!

so 192/3! = 32
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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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09 Aug 2015, 05:15
seekmba wrote:
If there are four distinct pairs of brothers and sisters, then in how many ways can a committee of 3 be formed and NOT have siblings in it?

A. 8
B. 24
C. 32
D. 56
E. 192

LET ABCD are Boys and PQRS are their sisters respectively
Case-1: All Boys – 4C3 = 4
Case-2: All Girl – 4C3 = 4
Case-3: 2 Boys and 1 girl – 4C2*2C1 = 12
Case-4: 2 Girl and 1 Boy – 4C2*2C1 = 12

Total Cases = 4+4+12+12 = 32

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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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26 Sep 2016, 22:06
AABBCCDD
so ABC can come or BCD or CDA
so 3! * 4 = 24
if A is not equal to A
then it becomes 24*2 = 48
which ones am I missing?
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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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26 Sep 2016, 23:40
paidlukkha wrote:
AABBCCDD
so ABC can come or BCD or CDA
so 3! * 4 = 24
if A is not equal to A
then it becomes 24*2 = 48
which ones am I missing?

Point 1: You can't take them as AABBCCDD because in sibling couple also first individual and second individuals are treated differently
Rather you should take them as A1A2 B1B2 C1C2 D1D2

In case you want to make 4 cases then
All Ones i.e. three of A1, B1, C1, D1 which can happen in 4C3 ways
All Twos i.e. three of A2, B2, C2, D2 which can happen in 4C3 ways
Two ones and one Two i.e. 4C2*2C1 = 12 (2C1 is used to select one Two out of remaining twos who are not siblings of Ones selected
Two Twos and one One i.e. 4C2*2C1 = 12 (2C1 is used to select one Two out of remaining twos who are not siblings of Ones selected

Total ways = 4+4+12+12 = 32

Point 2: you are using 3! in your solution which is completely redundant because there is no arrangement here. You only have to select 3 individuals out of 8 so the arrangement doesn't come in picture so use of 3! is completely incorrect on concept part

Point 3: I didn't understand why you used 4 in your solution.

I hope this helps!!!
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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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26 Sep 2016, 23:44
2
paidlukkha wrote:
AABBCCDD
so ABC can come or BCD or CDA
so 3! * 4 = 24
if A is not equal to A
then it becomes 24*2 = 48
which ones am I missing?

Another method to solve this question is

Select any three out of these 8 individuals = 8C3 ways

Subtract the unwanted cases i.e. cases in which 2 of 3 selected have one sibling pair which can be selected as 4*6
4 = number of ways to select one sibling pair i.e. two individuals
6 = No. of ways of selecting one out of 6 remaining individuals to make a group of 3 alongwith 2 selected in previous step

Total favourable cases = 8C3 - (4*6) = 56 - 24 = 32
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Re: If there are four distinct pairs of brothers and sisters, then in how  [#permalink]

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11 Jun 2018, 01:57
seekmba wrote:
If there are four distinct pairs of brothers and sisters, then in how many ways can a committee of 3 be formed and NOT have siblings in it?

A. 8
B. 24
C. 32
D. 56
E. 192

# of ways to from a committee of 3 with no siblings on it = (8 * 6 * 4)/3! = 32

The First person in the committee can be chosen in 8 ways.
The Second person in the committee can be chosen in 6 ways, since we will remove the sibling of first person selected & then choose.
The Third person in the committee can be chosen in 4 ways, since we will remove the sibling of second person selected, as well & then choose.

Now we are asked for the # of ways to form a committee & not the arrangement of the persons, hence order does not matter. To account for this we divide by 3!, which is the # of possible arrangements of 3 persons.

Thanks,
GyM
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Re: There are four distinct pairs of brothers and sisters. In  [#permalink]

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Re: There are four distinct pairs of brothers and sisters. In   [#permalink] 27 Jan 2020, 07:15
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