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Qudratic Equations

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by mmeital » Sun Jul 15, 2012 8:36 am
Hi,

This post is intended specifically for Manhattan GMAT instructors, although all replies are welcome!
I'm on day 9 of the 60 day study guide. This DS question is taken from the Manhattan GMAT Quadratic Equations Problem set - Algebra book, p.71 question 10:

What is X?
1. x=4y-4
2. xy=8

The explanation shows the correct answer as E. If we combine both statements, answer C, we reach 2 solutions for X. Why is that not acceptable?
Can anyone explain to me how the wording implies that the question we are answering is: Does X have only 1 solution? After all when we solve quadratic equations, 2 solutions are an acceptable starting point, unless we can eliminate of the solutions. Where in the wording of this problem are we to understand that only solution 1 can be acceptable?

I do not agree with the premise of the question, that only 1 solution is an acceptable answer and currently am sticking to C.

Please explain / confirm.

Thanks!

Matan
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by eagleeye » Sun Jul 15, 2012 9:02 am
mmeital wrote:Hi,

This post is intended specifically for Manhattan GMAT instructors, although all replies are welcome!
I'm on day 9 of the 60 day study guide. This DS question is taken from the Manhattan GMAT Quadratic Equations Problem set - Algebra book, p.71 question 10:

What is X?
1. x=4y-4
2. xy=8

The explanation shows the correct answer as E. If we combine both statements, answer C, we reach 2 solutions for X. Why is that not acceptable?
Can anyone explain to me how the wording implies that the question we are answering is: Does X have only 1 solution? After all when we solve quadratic equations, 2 solutions are an acceptable starting point, unless we can eliminate of the solutions. Where in the wording of this problem are we to understand that only solution 1 can be acceptable?

I do not agree with the premise of the question, that only 1 solution is an acceptable answer and currently am sticking to C.

Please explain / confirm.

Thanks!

Matan
Hi Matan:

For DS questions, whenever you have to determine the value of something, it must always be a single, unambiguous value. Even if we have two values, as in the case of a quadratic, we don't have "sufficient data" to choose between the two values. The only interpretation of every single GMAT question that asks for the value of something is "Can you find only a single value that satisfies the given data?".

:)
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by mmeital » Sun Jul 15, 2012 10:27 am
Hi Matan:

For DS questions, whenever you have to determine the value of something, it must always be a single, unambiguous value. Even if we have two values, as in the case of a quadratic, we don't have "sufficient data" to choose between the two values. The only interpretation of every single GMAT question that asks for the value of something is "Can you find only a single value that satisfies the given data?".

:)[/quote]

I went and reviewed the instruction in the OG13 and see your point. They write that for a value question "you must be able to find one, and only value."

Thanks!
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by tutorphd » Sun Jul 15, 2012 11:50 am
In VALUE DS questions, "sufficient" means producing exactly one numerical value.
Producing several numerical values or one symbolic value is not sufficient. For example, x=2 or 3 is not sufficient. Another example, x=b, is not sufficient.

In YES/NO DS questions, "sufficient" means answering the question with a definite YES, or a defnite NO. Beginners often do not realize that a definite NO answer is still sufficient.
Answering the question with ambiguous YES/NO i.e. sometimes with YES, sometimes with NO is not sufficient.

You should also know that questions of types "Is x=0 ?" are YES/NO questions because they can be answered with YES or NO. They are not value questions because the answer is not one numerical value of x.
Skype / Chicago quant tutor in GMAT / GRE
https://gmat.tutorchicago.org/
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