Of the 60 families in a certain neighborhood, 38 have a cat.

Question

Of the 60 families in a certain neighborhood, 38 have a cat. How many of the families in this neighborhood have a dog?

(1) 28 of the families in this neighborhood have a cat but not a dog
(2) The number of families in the neighborhood who have a dog and a cat is the same as the number of families who have neither a cat nor a dog.

Bunuel · Accepted Answer

Good question. +1 to calreg11.

Mike is right that a double-matrix method is probably the easiest way to solve this problem and kys123 is right that the answer to the question is B (+1).

Of the 60 families in a certain neighborhood, 38 have a cat. How many of the families in this neighborhood have a dog?

Consider matrix below. Numbers in black are given and numbers in red are calculated.

Attachment:

Stem.PNG [ 2.53 KiB | Viewed 25636 times ]

(1) 28 of the families in this neighborhood have a cat but not a dog.

Attachment:

Statement 1.PNG [ 2.68 KiB | Viewed 25613 times ]

So you can see that we can no way get # of the families in this neighborhood who has a dog (? in the matrix). Not sufficient.

(2) The number of families in the neighborhood who have a dog and a cat is the same as the number of families who have neither a cat nor a dog.

Attachment:

Statement 2.PNG [ 2.94 KiB | Viewed 25552 times ]

You can see that if # of families who have a dog and a cat and # of families who have neither a cat nor a dog is x, then # of families who has cat but not dog is 38-x. Next, total # of families who has no dog is (38-x)+x=38 and # of families who has a dog is 60-38=22. Sufficient.

Answer: B.

Hope it helps.

mikemcgarry · Answer

Hi, there. I'm happy to help.

I solved this with a double-matrix method --- because that can get sloppy in the plaintext of these posting windows, I created a pdf attachment. The double matrix method is a tremendously powerful method for solving these overlapping set problems. At Magoosh, we have a whole series of video lessons going over everything you need to know for GMAT math, including one that explains exactly how to set up the double matrix method of solution. Please let me know if you should have any questions. Mike

kys123 · Answer

Finally get why it's B.
Total residence = 60. 
Those with no pets = X
Those with cats = 38. Those with cats only = 38 -x
With cats n dogs = x.
Dogs = D. Dogs only = D - x

Total residence with pet = Dogs + Cats only.
60 - x = Cats only (38 - x) + D. The reason for doing this is because total amount of pet owners is people with cats + people with dogs plus people with both. If you add total # of dog owners plus total # of cat owners together your adding owners of both pets twice.

Therefore 60 - X + X -38 = D. 
D = 22.

B only is significant

mikemcgarry · Answer

It's true that (# with 1+ pets) = (# with cats only) + (# with dogs only) + (number with both) Using the notation you adopted, 60 - x = (38 - x) + D + x ---> you forgot that last term . 60 - x + x - 38 - x = D 22 - x = D And, thus, we cannot establish the value of D with knowing the value of x, so Statement #2, by itself, is insufficient . As I show in the pdf posted above, the answer is C Please let me know if you have any questions. Mike

kys123 · Answer

Okay, but answer still true. D who only only dogs. Individuals who only own dogs plus individual who only own cats = Total individual who own dogs. That's what we're trying to find, so therefore B is correct.
22 = Number in the neighbourhood with a dog (D   +X  .

Same equation you stated

kys123 · Answer

This is what I did. The way that Bunuel solve this problem was a lot more elegant, but for me my way is more intuitive. I know everything inside the matrix should add to 60. Hence my solution.

mikemcgarry · Answer

My apologies to kys123

The solution given by Bunuel & kys123 is perfectly correct.

I realize I was misreading/misinterpreting the question, thinking it was asking for the number of people who owned only a dog, i.e. a dog and no cat, not simply the number of dog owners. A good reminder how crucial careful reading is.

If the question were asking for the people who owned only a dog, the answer would be C.

As it stands, though, with the question asking for the number of people who own only a dog, the answer is clearly B, as Bunuel and kys123 have shown.

Again, my apologies for any confusion.

Mike

thevenus · Answer

very very tricky, Bunuel's response is fantastic(as usual

) well, (i) is insufficient (ii) is sufficient , here is the catch; as we all know that A+B=C so, A=C-B & B=C-A Now check out the file attached

KarishmaB · Answer

Yes, the question is really good. I like to show a series of diagrams to my students to explain what the statement 'number of families with both = number of families with none' implies. It means the sum of number of families with cat and number of families with dog is constant and is equal to 60. For every one family that has both, there is a family that has none (to keep their numbers equal) Look at the diagrams below. If the number of families that have neither a dog nor a cat is 0, the number of families with a dog is 60 - 38 = 22. Now what happens when you overlap one family? There is one family which has neither a cat nor a dog. The number of families with a cat or a dog or both reduces by 1 and the number of families with neither increases by 1. The sum is kept constant at 60. The following diagrams should make it clear. Ques3.jpg Ques4.jpg Ques5.jpg

LalaB · Answer

38+x+Neither-Both=60
Neither=Both

38+x=60
x=22

tk1tez7777 · Answer

Its strange that a tutor got it wrong and lot of students have got it right

sanket1991 · Answer

If we use Venn Diagram it's a lot faster and space saver. Even though, because of timer, I misread question and thought 'the examiner asked about families with just a dog' and marked wrong answer :beatup

shasadou · Answer

indeed Venn + formula solve Statement 2 in 15 secs - very straightforward. on the overlapping sets very often matrix is the best approach however this question is the case when Venn is the shortest solution

MathRevolution · Answer

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution

Of the 60 families in a certain neighborhood, 38 have a cat. How many of the families in this neighborhood have a dog?

(1) 28 of the families in this neighborhood have a cat but not a dog
(2) The number of families in the neighborhood who have a dog and a cat is the same as the number of families who have neither a cat nor a dog.

This is a “2-by-2” question, one of the most common types of questions in GMAT math. From modifying the original condition and the question, we can obtain a table below.

From the table, we can see that the question is a+b=? There are 4 variables (a,b,c, and d) and 2 equations (a+b+c+d=60 and a+c=38). In order to match the number of variables and the number of equations, we need 2 more equations. Since the condition 1) and 2) each has 1 equation, there is high chance that C is the answer.

Using both the condition 1) and 2), we get c=28. Also, since a=d, we get a+c=d+c=a+28=d+28=38. Then, a=d=10. So, if we substitute in a+b+c+d=60, we get 10+b+28+10=60. Then, b=12. So, a+b=10+12=22. So, the answer is unique and the condition is sufficient. So we can see how C could be the answer.

However, a question involving hidden integer is one of key questions (integer, statistics, inequality, probability, absolute value) and we have to consider Mistake Type 4(A).

In the case of the condition 2), since a=d, we get a+c=d+c=38. Then, if we substitute it into a+b+c+d=60, we get a+b+38=60. So, a+b=22. The answer is unique and the condition is sufficient. If both C and B are the answers to the question, the final correct answer choice is B.

For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.

Attachments

GCDS calreg11 Of the 60 families in a certain (20160103).jpg [ 23.73 KiB | Viewed 18903 times ]

BrainLab · Answer

I've also solved this one easily using double-matrix. When you have 2 terms use always double-matrix, if you have >2 use formulas.

MathRevolution · Answer

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution

Of the 60 families in a certain neighborhood, 38 have a cat. How many of the families in this neighborhood have a dog?

(1) 28 of the families in this neighborhood have a cat but not a dog
(2) The number of families in the neighborhood who have a dog and a cat is the same as the number of families who have neither a cat nor a dog.

This is a “2-by-2” question, one of the most common types of questions in GMAT math. From modifying the original condition and the question, we can obtain a table below.

From the table, we can see that the question is a+b=? There are 4 variables (a,b,c, and d) and 2 equations (a+b+c+d=60 and a+c=38). In order to match the number of variables and the number of equations, we need 2 more equations. Since the condition 1) and 2) each has 1 equation, there is high chance that C is the answer.

Using both the condition 1) and 2), we get c=28. Also, since a=d, we get a+c=d+c=a+28=d+28=38. Then, a=d=10. So, if we substitute in a+b+c+d=60, we get 10+b+28+10=60. Then, b=12. So, a+b=10+12=22. So, the answer is unique and the condition is sufficient. So we can see how C could be the answer.

However, a question involving hidden integer is one of key questions (integer, statistics, inequality, probability, absolute value) and we have to consider Mistake Type 4(A).

In the case of the condition 2), since a=d, we get a+c=d+c=38. Then, if we substitute it into a+b+c+d=60, we get a+b+38=60. So, a+b=22. The answer is unique and the condition is sufficient. If both C and B are the answers to the question, the final correct answer choice is B.

For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.

Goldenfuture · Answer

Can be solved mentally - here it goes 
60 = Cat + Only Dog (since overlap is added in Cat) + none
Statement 1 only gives us what this overlap is not none part => okay 10 have both - I don't know none so I can't solve this

Statement II Says overlap = none = x
so 60 = Only Cat + Overlap + Only Dog + None 
 60 = 38 + Dog + X (which is overlap)
22 = Only dOG + OVERLAp - answer in 1.5 min

Of the 60 families in a certain neighborhood, 38 have a cat.

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Of the 60 families in a certain neighborhood, 38 have a cat.

Prep Toolkit

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

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From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

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