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dabaobao
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A group of 10 people consists of 2 married couples and 6 bachelors. A committee of 4 is to be selected from the 10 people. How many different committees can be formed if the committee can consist of at most one married couple?

A) 95
B) 155
C) 185
D) 209
E) 210

Official Solution


Credit: Veritas Prep

We have to select 4 people out of: 6 bachelors and 2 married couples.

The number of ways of selecting any 4 people out of 10 is 10*9*8*7/4! = 210 (Note here that we are just selecting 4 people. We are not arranging them so we divide by 4!)

The people will get selected in various ways:

1. Four bachelors
2. One from a couple and three bachelors
3. Two from two different couples and two bachelors
4. One couple and two bachelors
5. One couple, one person from a couple, one bachelor
6. Two couples

If we add the number of committees possible in each of these cases, we will get 210. Out of all these cases, only the last one (two couples) has more than one married couple. Instead of calculating the number of different committees that can be formed in each of the first five cases, we can calculate the number of committees in the last case and subtract it from 210.

How many different committees can be formed such that there are 2 couples? Only one since we have only 2 couples. We will have to select both the couples and we will get 4 people.

Number of different committees of 4 people such that there is at most one married couple = 210 – 1 = 209.

Just for practice, let’s see how we can calculate the different number of committees that can be formed in each of the first five cases. The sum of all these cases should give us 209.

1. Select 4 bachelors from 6 bachelors in 6*5*4*3/4! = 15 different committees
2. Select 1 person out of the two couples (4 people) in 4 ways and 3 bachelors from 6 bachelors in 6*5*4/3! = 20 ways. So you select the 4 people in 4*20 = 80 different committees
3. Select 2 people from 2 different couples in 4*2/2! = 4 ways and 2 bachelors from 6 bachelors in 6*5/2! = 15 ways. So you select the 4 people in 4*15 = 60 different committees
4. Select 1 couple in 2 ways and 2 bachelors from 6 bachelors in 6*5/2! = 15 ways. So you select the 4 people in 2*15 = 30 different committees
5. Select 1 couple in 2 ways, 1 person from the remaining couple in 2 ways and 1 bachelor from 6 bachelors in 6 ways. So you can select the 4 people in 2*2*6 = 24 different committees

The sum of all these five cases = 15 + 80 + 60 + 30 + 24 = 209 different committees (as expected)
ANSWER: D

I think you can combine case 3 and 4. As long as we are choosing only 2 people from the 4 married individuals, we're not violating the restriction.
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Dear all:

I wanted to inquire why I don't get the same answer of 209 under my methodology. Please note:

M1F1
M2F2
B1,B2,B3,B4,B5,B6

10C3 = (10x9x8x7)/(4x3x2x1) = 210

To calculate the number of combinations for groups of 2 couples:

(4x1)/(2x1) x (2x1)/(2x1) = 2

where the 4 in the numerator represents the number of options to select the first person for the first couple, 1 represents the option for the second person and where the 2 in the numerator represents the number of options to select the third person as the first candidate for the 2nd couple. We divide by 2x1 as we are calculating groups of 2.

210 - 2 = 208

I get the same answer using the combinations formula, but wanted to clarify why I was erring by 1 under this adaptation of the fundamental counting principle. Thank you.
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imslogic
Dear all:

I wanted to inquire why I don't get the same answer of 209 under my methodology. Please note:

M1F1
M2F2
B1,B2,B3,B4,B5,B6

10C3 = (10x9x8x7)/(4x3x2x1) = 210

To calculate the number of combinations for groups of 2 couples:

(4x1)/(2x1) x (2x1)/(2x1) = 2

where the 4 in the numerator represents the number of options to select the first person for the first couple, 1 represents the option for the second person and where the 2 in the numerator represents the number of options to select the third person as the first candidate for the 2nd couple. We divide by 2x1 as we are calculating groups of 2.

210 - 2 = 208

I get the same answer using the combinations formula, but wanted to clarify why I was erring by 1 under this adaptation of the fundamental counting principle. Thank you.


Hello,

If we are given there are these 4 people that need to be selected to form a committee of 4, so ideally you ask yourself in how many ways can this be done?
1. Right? You will ask all four of them to join the committee.

Rather than going into details of PnC, think logically how would you select these 4 designated set of people (who are a part of 2 couples AB,CD) into a committee of 4 people as is.
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Clearly it's easiest to do the Total minus the number of committees that do have 2 married couples:

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How 1 Came?? ( A committee of 4 with 2 couples).....Can You Please Elaborate?
Akshat1994
The total ways in which a committee of 4/ 10 can be formed = 10C4

The no: of ways in which a committee can be formed with atmost one couple= Total ways- ( A committee of 4 with 2 couples)

Answer: 10C4-1= 209

Posted from my mobile device
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Chanchal2809
How 1 Came?? ( A committee of 4 with 2 couples).....Can You Please Elaborate?
Akshat1994
A group of 10 people consists of 2 married couples and 6 bachelors. A committee of 4 is to be selected from the 10 people. How many different committees can be formed if the committee can consist of at most one married couple?

A) 95
B) 155
C) 185
D) 209
E) 210

The total ways in which a committee of 4/ 10 can be formed = 10C4

The no: of ways in which a committee can be formed with atmost one couple= Total ways- ( A committee of 4 with 2 couples)

Answer: 10C4-1= 209

Posted from my mobile device

When selecting 4 people out of 10, only one combination includes both married couples. Since we are interested in all combinations except the one with two married couples, we subtract that single case. Thus, 10C4 - 1 gives the number of 4-person committees without two married couples.
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dabaobao

Given: A group of 10 people consists of 2 married couples and 6 bachelors. A committee of 4 is to be selected from the 10 people.

Asked: How many different committees can be formed if the committee can consist of at most one married couple?

Total ways to form a committee of 4 out of 10 people = 10C4 = 210

Number of ways to form a committee if the committee consists of both married couple = 1

Number of ways to form a committee if the committee can consist of at most one married couple = 210 - 1 = 209

IMO D
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KarishmaB
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I have a doubt.

"Select 2 people from 2 different couples in 4*2/2! = 4 ways and 2 bachelors from 6 bachelors in 6*5/2! = 15 ways. So you select the 4 people in 4*15 = 60 different committees"
I understand the 4*2/2! as using the fundamental principle of counting and then "un-arranging"

BUT what am I doing wrong when I apply the similar logic in below statement?
"Select 1 couple in 2 ways, 1 person from the remaining couple in 2 ways and 1 bachelor from 6 bachelors in 6 ways. So you can select the 4 people in 2*2*6 = 24 different committees"

My Logic: _ _ _ (three blanks to be filled by 3 ppl among the couples)

4*1*2 should be the total ways of arranging
and hence 4*1*2/3! should be ways of selecting, I know this is wrong but can you tell why exactly is it wrong??
Furthermore if I include bachelor in this itself 4*1*2*6/4! what is wrong with this?

Thanks in advance!!!:)
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