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Data sufficiency, the answer "C"

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by Mr.Hollywood » Wed Dec 21, 2011 8:15 pm
I actually have a question about I would say how to eliminate the answer: "C" in Data Sufficiency questions. Here is an example:

What is the value of x^2-y^2?
1) x-y=y+2
2) x-y=1/x+y

I do know the answer is A and also understand how A was derived. My question is when 1) is sufficient, do we still need to look at answer "C" to see weather 1) and 2)are sufficient together?
Also, under what sort of circumstance answer C needs to be chosen.

I'm embarrassed for asking a such low level question, but better understand it than cry on the test day. Thank you so much for your time guys.
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Source: — Data Sufficiency |
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by karthikpandian19 » Wed Dec 21, 2011 9:23 pm
There are two things:

Firstly, i am sure the answer is "B" and not A

Secondly, coming to your question:
Adopt this strategy:
write in your worksheet before start of the problem with
AD
BCE (if you are going to start with 1st sentence)

or

BD
ACE (if you are going to start with 2nd sentence)

1. Now for eg. we can take you are starting with 1st sentence, then
AD
BCE
now from the first sentence, you can find its sufficiency:
1a) If Sufficient then, cancel the BCE

it remains "AD"

1b) If Not Sufficient then, cancel AD

it remains "BCE"

2. now start the second sentence, you can find its sufficiency:
2a) If sufficient & with 1a) condition, then cancel A

it remains "D"

2a) If sufficient & with 1b) condition, then cancel CE

it remains "B"

2b) If Not Sufficient then, cancel B

it remains "CE"

3) Now the last condition is if "CE" remains then combine the statements and check for sufficiency

3a) If Sufficient, then cancel E

it remains "C"

3b) If Not sufficient, then cancel C
it remains "E"

If you still dont understand then try seeing the Flow chart in OG
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by Ian Stewart » Thu Dec 22, 2011 1:23 am
Mr.Hollywood wrote: My question is when 1) is sufficient, do we still need to look at answer "C" to see weather 1) and 2)are sufficient together?
Also, under what sort of circumstance answer C needs to be chosen.
Answer C on DS questions reads as follows:

BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

Notice the highlighted part: you *only* consider C (and therefore *only* use both statements together) if neither statement is sufficient alone. So if you discover that Statement 1 is sufficient alone when solving a problem, then C will never be the correct answer - the answer can only be A or D.



And for the question you quoted, I think you meant to say that the answer is B.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by sam2304 » Thu Dec 22, 2011 2:01 am
IMO B.

Use ADBCE approach as mentioned by karthikpandian. Don't choose C unless you couldn't answer the question with A/B alone.
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by Mr.Hollywood » Thu Dec 22, 2011 9:55 pm
Clear as crystal! Thank you!
Ian Stewart wrote:
Mr.Hollywood wrote: My question is when 1) is sufficient, do we still need to look at answer "C" to see weather 1) and 2)are sufficient together?
Also, under what sort of circumstance answer C needs to be chosen.
Answer C on DS questions reads as follows:

BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

Notice the highlighted part: you *only* consider C (and therefore *only* use both statements together) if neither statement is sufficient alone. So if you discover that Statement 1 is sufficient alone when solving a problem, then C will never be the correct answer - the answer can only be A or D.



And for the question you quoted, I think you meant to say that the answer is B.
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by user123321 » Fri Dec 23, 2011 6:18 am
shouldn't the question say,
x!=y in second condition??
any thoughts anyone?
can we expect similar errors in real gmat exam?

Thanks
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by shankar.ashwin » Fri Dec 23, 2011 9:38 am
Does it have to?

if x=y, we have

0 = 1/x+y

0=1 ? that would not be possible no.?
user123321 wrote:shouldn't the question say,
x!=y in second condition??
any thoughts anyone?
can we expect similar errors in real gmat exam?

Thanks
user123321
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by user123321 » Fri Dec 23, 2011 11:43 am
shankar.ashwin wrote:Does it have to?

if x=y, we have

0 = 1/x+y

0=1 ? that would not be possible no.?
user123321 wrote:shouldn't the question say,
x!=y in second condition??
any thoughts anyone?
can we expect similar errors in real gmat exam?

Thanks
user123321
Sorry that's a typo. I forgot to put the -ve sign before y.
i meant to say x!=-y in the second option.
because 1/(x+y) was given and if x = -y then that equation becomes invalid.
If same thing happens in actual GMAT, do we need to assume it as a typo or need to use common sense and proceed?

Thanks
user123321
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by shankar.ashwin » Fri Dec 23, 2011 9:42 pm
user123321 wrote:
Sorry that's a typo. I forgot to put the -ve sign before y.
i meant to say x!=-y in the second option.
because 1/(x+y) was given and if x = -y then that equation becomes invalid.
If same thing happens in actual GMAT, do we need to assume it as a typo or need to use common sense and proceed?

Thanks
user123321
Hi, I think there's a important distinction here.

Statement 2 is always considered to hold TRUE, so its understood here 'x' cannot be equal to '-y', because in that case Statement 2 would not hold true.

But I get your question, IMO whatever official problems I have seen explicitly state conditions, which would not make the inequality/equation undefined (like denominator becomes 0 and so on). In general, notice PS problems usually come with such statements.

But incase of DS, statement 2 is already TRUE for all values of 'x' and 'y', so its safe for us to assume denominator cannot be 0.

I think GMAC puts in a lot of effort and would not come up with such glaring/obvious errors. As for this problem, I don't think its a typo, seems like a legit /possible question without ambiguity.

Just my thoughts, maybe 'Experts' could clarify our doubt.
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