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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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dave13 wrote:
VeritasKarishma wrote:
sjung92 wrote:
Why is everybody adding both instead of subtracting? Official GMAT also adds both and I'm confused as to why.

I thought the formula was Total = A + B - Both + Neither


A different formula is being used here:

Total = Only A + Only B + Both + Neither

In the formula given by you, A includes Both and B also includes Both so we subtract out Both once.
In the formula being used in this question, we are considering those who have dogs only and those who have cats only. So we add to them those who have both and those who have neither to give us the total number of people.


VeritasKarishma could you pls slightly tweak :grin: this DS question into the one where we would need to use Total = A + B - Both + Neither


Normally, the questions ask for number of households having at least one cat or at least one dog.
That is n(C or D)

"Total - Neither" is "at least one cat or at least one dog".

Total = n(C or D) + Neither
n(C or D) = n(C) + n(D) - Both

Look at what this question is asking and that is what makes it special:

Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

It just wants you to remove Both from n(C or D).

n(C or D) - Both = n(C) + n(D) - Both - Both
n(C or D) - Both = n(C) - Both + n(D) - Both
n(C or D) - Both = n(Only C) + n(Only D)

That is how the two are equivalent.

Use your std formula if you wish.

Total = n(C or D) + Neither
50 = n(C or D) + 14
n(C or D) = 36
This includes those households that have both. So remove Both
n(C or D) - Both = 36 - 4 = 32
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.



The general formula of two overlapping sets is ; TOTAL = A + B -BOTH + NEITHER

At least one for two overlapping sets:

TOTAL - BOTH + NEITHER= AT LEAST CATS OR DOGS

1. BOTH IS GIVEN BUT NO INFORMATION ABOUT NEITHER; INSUFF.

2. NEITHER ' 14 IS GIVEN BUT BOTH IS ABSENT; INSUFF.

1 & 2 TOGATHER IS SUFFICIENT. ANS C.
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Expert Reply
INSEADIESE wrote:
VeritasKarishma Bunuel chetan2u GMATBusters nick1816

ScottTargetTestPrep

Hi Experts!

Hope you all are doing well

Although i am able to understand that we need both "neither" and "both" so as to find what has been asked in this question, I have a doubt..

For point (2) of the question, Isn't "not A and not B" equal to "not(A and B)", which basically means "not(Both A and B)", which definitely isn't the same as "Neither A nor B" ????

Looking forward to hearing from you

Best Regards,


"not A and not B" is not the same thing as "not(A and B)". In order for "not A and not B" to hold, both "not A" and "not B" must hold; in other words, your element should not belong to A and should not belong to B. In a two-set Venn diagram, this would correspond to the area outside the two sets, which is usually denoted as "Neither". So, "not A and not B" is actually the same thing as "Neither A nor B".

On the other hand, "not(A and B)" is true when your element is not in "A and B", meaning your element either isn't in A or isn't in B. As long as the element misses at least one of the sets, your element is not in "A and B" (which is the same thing as saying your element is in "not(A and B)"). In a two-set Venn diagram, this would correspond to the area outside the overlap of the two sets.
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
ScottTargetTestPrep wrote:
Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.


We have a group of 50 households and need to determine how many of those households have at least one cat or at least one dog, but not both.

We can use the following formula:

total = # with at least one dog only + # with at least one cat only + # with both + # with neither

50 = # with at least one dog only + # with at least one cat only + # with both + # with neither

So, we need to determine # with at least one dog only + # with at least one cat only

Statement One Alone:

The number of households that have at least one cat and at least one dog is 4.

So, we have:

50 = # with at least one dog only + # with at least one cat only + 4 + # with neither

46 = # with at least one dog only + # with at least one cat only + # with neither

Since we don’t know the # with neither, we can not determine # with at least one dog only + # with at least one cat only. Statement one alone is not sufficient to answer the question.

Statement Two Alone:

The number of households that have no cats and no dogs is 14.

So, we have:

50 = # with at least one dog only + # with at least one cat only + # with both + 14

36 = # with at least one dog only + # with at least one cat only + # with both

Since we don’t know the # with both, we cannot determine # with at least one dog only + # with at least one cat only. Statement two alone is not sufficient to answer the question.

Statements One and Two Together:

Using statements one and two, we have:

50 = # with at least one dog only + # with at least one cat only + 4 + 14

50 = # with at least one dog only + # with at least one cat only + 18

32 = # with at least one dog only + # with at least one cat only

Answer: C




Why did we use the formula
total= cat+dog+both+neither

whereas the formula is : Total- neither= cat+dog-both+neither?????

can you please explain?
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.



can you please explain why did we use the formula

Total = cat+dog+both+neither

whereas the formula is

total = cat+ dog -both+neither
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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ManyataM wrote:
Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.



can you please explain why did we use the formula

Total = cat+dog+both+neither

whereas the formula is

total = cat+ dog -both+neither


ScottTargetTestPrep Please answer.
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Expert Reply
ManyataM wrote:
Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.



can you please explain why did we use the formula

Total = cat+dog+both+neither

whereas the formula is

total = cat+ dog -both+neither



The formula depends on what cat + dog means.
If it is only, then only cat + only dog + both + neither.
And that is exactly what at least one cat or at least one dog but not both means, so we look for only cat + only dog.

We use the formula c+d-both+neither, when c includes all households that is only cat + both cat and dog.
As both gets added twice we subtract it once.

Both the formula mean the same depending on what c and d include.
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Expert Reply
ManyataM wrote:
ScottTargetTestPrep wrote:
Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.


We have a group of 50 households and need to determine how many of those households have at least one cat or at least one dog, but not both.

We can use the following formula:

total = # with at least one dog only + # with at least one cat only + # with both + # with neither

50 = # with at least one dog only + # with at least one cat only + # with both + # with neither

So, we need to determine # with at least one dog only + # with at least one cat only

Statement One Alone:

The number of households that have at least one cat and at least one dog is 4.

So, we have:

50 = # with at least one dog only + # with at least one cat only + 4 + # with neither

46 = # with at least one dog only + # with at least one cat only + # with neither

Since we don’t know the # with neither, we can not determine # with at least one dog only + # with at least one cat only. Statement one alone is not sufficient to answer the question.

Statement Two Alone:

The number of households that have no cats and no dogs is 14.

So, we have:

50 = # with at least one dog only + # with at least one cat only + # with both + 14

36 = # with at least one dog only + # with at least one cat only + # with both

Since we don’t know the # with both, we cannot determine # with at least one dog only + # with at least one cat only. Statement two alone is not sufficient to answer the question.

Statements One and Two Together:

Using statements one and two, we have:

50 = # with at least one dog only + # with at least one cat only + 4 + 14

50 = # with at least one dog only + # with at least one cat only + 18

32 = # with at least one dog only + # with at least one cat only

Answer: C




Why did we use the formula
total= cat+dog+both+neither

whereas the formula is : Total- neither= cat+dog-both+neither?????

can you please explain?


So, the following formula exists for a two category overlapping sets problem:

Total = Only A + Only B + both + neither
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Firstly we know, Total member=50.
Formula used= (A or b)= (A)+(B)-(A and B).
OR
Total= None+(A or b)+ (A and B)

(1) The number of households that have at least one cat and at least one dog is 4.
From 1 we get (A and B)=4 But only statement 1 is not sufficient to find (A or b).

(2) The number of households that have no cats and no dogs is 14.
Now we know Total=50 and none= 14. But we still have no clue about (A and B) and (A or B)

COMBINING BOTH
Total= None+(A or b)+ (A and B)
50=14+ (A or b)+4
(A or b)=50-14-4=38

:)
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Firstly we know, Total member=50.
Formula used= (A or b)= (A)+(B)-(A and B).
OR
Total= None+(A or b)+ (A and B)

(1) The number of households that have at least one cat and at least one dog is 4.
From 1 we get (A and B)=4 But only statement 1 is not sufficient to find (A or b).

(2) The number of households that have no cats and no dogs is 14.
Now we know Total=50 and none= 14. But we still have no clue about (A and B) and (A or B)

COMBINING BOTH
Total= None+(A or b)+ (A and B)
50=14+ (A or b)+4
(A or b)=50-14-4=38
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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Video solution from Quant Reasoning:
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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VeritasKarishma wrote:
Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.


It is fairly simple.

You have 50 households. No of households with no cats and no dogs = 14.
So 50 - 14 = 36 households have at least one cat or at least one dog or both.

So 36 households lie in the two overlapping circles, some in yellow region (only at least 1 cat), some in blue (only at least 1 dog) and 4 in green (both).

Attachment:
Screenshot 2019-03-31 at 11.50.25.png


Question: "how many have at least one cat or at least one dog, but not both?"
There are 4 households in both region. So number of households in yellow + blue only = 36 - 4 = 32


Quote:
hello expert,

do't we need to deduct 4 two times in order to get number of household only in blue and orange?


50 - 14 = 36 gives you n(C or D)

Note that n(C or D) = n(C) + n(D) - Both = 36

So already 1 Both has been subtracted out and we got 36. Now to remove Both, all we need to do is subtract it out once more only.
You get 36 - 4 = 32
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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Bunuel wrote:
Of a group of 50 households, how many have at least one cat or at least one dog, but not both?

(1) The number of households that have at least one cat and at least one dog is 4.
(2) The number of households that have no cats and no dogs is 14.


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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
avigutman wrote:
Video solution from Quant Reasoning:
Subscribe for more: https://www.youtube.com/QuantReasoning? ... irmation=1



avigutman, could you please explain me why Statement I means that 4 is the value for both cats and dogs?

When I first attempted the question, I placed it as the sum of (Both Cat and Dog) + (Cat, but not Dog) +( Dog, but not Cat).

Will appreciate your input!
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
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AntonioGalindo wrote:
could you please explain me why Statement I means that 4 is the value for both cats and dogs?

When I first attempted the question, I placed it as the sum of (Both Cat and Dog) + (Cat, but not Dog) +( Dog, but not Cat).



AntonioGalindo consider these two statements:
1. The number of households that have at least one cat and at least one dog is 4.
2. The number of households that have at least one cat or at least one dog is 4.

For the first statement (the original statement from the problem), in order to be counted, you have to have:
at least one cat and at least one dog
For the second statement, in order to be counted, you have to have:
at least one cat or at least one dog is 4

Please let me know if you need further explanation.
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
avigutman wrote:
AntonioGalindo wrote:
could you please explain me why Statement I means that 4 is the value for both cats and dogs?

When I first attempted the question, I placed it as the sum of (Both Cat and Dog) + (Cat, but not Dog) +( Dog, but not Cat).



AntonioGalindo consider these two statements:
1. The number of households that have at least one cat and at least one dog is 4.
2. The number of households that have at least one cat or at least one dog is 4.

For the first statement (the original statement from the problem), in order to be counted, you have to have:
at least one cat and at least one dog
For the second statement, in order to be counted, you have to have:
at least one cat or at least one dog is 4

Please let me know if you need further explanation.


Thank you Avi! Makes sense! Appreciate your valuable help.
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Re: Of a group of 50 households, how many have at least one cat or at [#permalink]
Statement 1 says "atleast 1 cat and atleast 1 dog" . It means both Cat and dog,
Thus-
Dog Dog(not Total
Cat 4 b
Cat(not) a ?
Total 50
​We need to find a+b. ​Not sufficient.

Statement 2 says - "No cats and dogs"
Dog Dog(not Total
Cat ? b
Cat(not) a 14
Total 50
​We need to find a+b. ​Not sufficient.

​Combining these two,

Dog Dog(not Total
Cat 4 b 4+b
Cat(not) a 14 14+a
Total 4+a 14+b 50

​Now, 4+a + 14 + b = 50
a+b = 32
Sufficient.

Answer: C
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