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What is the value of x?

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Shalabh09
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What is the value of x? [#permalink] New post   01 May 2015, 06:32
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59% (01:42) correct 41% (01:52) wrong based on 161 sessions
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What is the value of x?

(1) \(x^2+x+10=16\)
(2) \(x=4y^4+2y^2+2\)

Could an Expert please help with the difficulty level as well? I'm not sure if this falls in 600-700.
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Bunuel
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Re: What is the value of x? [#permalink] New post   01 May 2015, 07:23
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Shalabh09 wrote:
What is the value of x?

(1) \(x^2+x+10=16\)
(2) \(x=4y^4+2y^2+2\)

Could an Expert please help with the difficulty level as well? I'm not sure if this falls in 600-700.


What is the value of x?

(1) \(x^2+x+10=16\) --> \(x^2+x-6=0\) --> x=-3 or x=2. Not sufficient.

(2) \(x=4y^4+2y^2+2\). Without knowing the value of y we cannot get the value of x. Not sufficient.
Notice that from this statement it follows that x must be positive: \(x=4y^4+2y^2+2=(nonnegative)+(nonnegative)+(positive)=positive\).

(1)+(2) Since from (2) x is positive, then from (1) we are left with x=2. Sufficient.

Answer: C.

P.S. As for the difficulty level, I'd say it's ~550-600 level question.
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Re: What is the value of x? [#permalink] New post   01 May 2015, 15:09
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Hi Shalabh09,

The GMAT often finds ways to test you on concepts that you know, but in ways that you're not used to thinking about. This question isn't too difficult (I'd rank it in the low-600s), but it does test you on Quadratics in ways that you're probably not used to thinking about.

Bunuel's explanation showcases the necessary "math" involved, so I won't rehash any of that here. Instead, I'll talk about the 'patterns' that you're supposed to spot in the construction of this question.

First, we're asked for the value of X. This is a standard question to ask in DS.

Second, Fact 1 gives us what appears to be a Quadratic equation with some extra "math" thrown in. You're probably used to solving Quadratic equations when they are set equal to 0..... so do the necessary math to set THIS equation equal to 0 and solve. You'll find that there are 2 solutions: X = -3 and X = 2. Fact 1 is INSUFFICIENT

Third, Fact 2 shows us that X is based on the value of Y. Since Y could be ANY value, there are multiple values for X. Fact 2 is INSUFFICIENT. There IS another thing worth noting about Fact 2 though...Notice the "even powers".....no matter what value you plug in for Y, the value of X will ALWAYS be POSITIVE.....

Combined, we know.....
X = -3 or X = 2
X will ALWAYS be positive

Combined, SUFFICIENT

Final Answer:
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C


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What is the value of x? [#permalink] New post   03 May 2015, 07:27
Bunuel wrote:
Shalabh09 wrote:
What is the value of x?

(1) \(x^2+x+10=16\)
(2) \(x=4y^4+2y^2+2\)

Could an Expert please help with the difficulty level as well? I'm not sure if this falls in 600-700.


What is the value of x?

(1) \(x^2+x+10=16\) --> \(x^2+x-6=0\) --> x=-3 or x=2. Not sufficient.

(2) \(x=4y^4+2y^2+2\). Without knowing the value of y we cannot get the value of x. Not sufficient.
Notice that from this statement it follows that x must be positive: \(x=4y^4+2y^2+2=(nonnegative)+(nonnegative)+(positive)=positive\).

(1)+(2) Since from (2) x is positive, then from (1) we are left with x=2. Sufficient.

Answer: C.

P.S. As for the difficulty level, I'd say it's ~550-600 level question.


Hi Bunuel

There is no information given about Y , Y can be a complex number too, so shouldn't i choose answer E ?

The roots of the polynomial \(4y^4+2y^2+5=0\) are:
\(Y_1=0.6588+0.82705∗i\)
\(Y_2=0.6588−0.82705∗i\)
\(Y_3=−0.6588+0.82705∗i\)
\(Y_4=−0.6588−0.82705∗i\)

Generally i have seen that GMAT questions always give hints about the numbers, in this question nothing has been told about Y,Strange.

noticed that there are a few questions floating in gmatclub , testing knowledge on complex numbers...
eg : if-the-square-of-the-sum-of-the-roots-of-a-quadratic-equation-197205.html
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Re: What is the value of x? [#permalink] New post   03 May 2015, 08:08
Hi Bunuel & EMPOWERgmatRichC,

Thank you very much for your detailed explanations. I guess it was just a new type of question so startled me a bit. Once the catch, which you both have shown, is seen it becomes quite simple.

Moreover, I guess due to these gaps I'm still stuck at Q41/42. I need some resources to practice such concepts. Could you guide me in this direction too?

Lucky : I read in one other forum, GMAT always deals with real numbers. So, there is no concept of complex numbers here. Hope this helps as well.

Thank you again.

Best Regards,
Shalabh.
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What is the value of x? [#permalink] New post   03 May 2015, 08:28
Shalabh09 wrote:
Hi Bunuel & EMPOWERgmatRichC,

Thank you very much for your detailed explanations. I guess it was just a new type of question so startled me a bit. Once the catch, which you both have shown, is seen it becomes quite simple.

Moreover, I guess due to these gaps I'm still stuck at Q41/42. I need some resources to practice such concepts. Could you guide me in this direction too?

Lucky : I read in one other forum, GMAT always deals with real numbers. So, there is no concept of complex numbers here. Hope this helps as well.

Thank you again.

Best Regards,
Shalabh.


i am aware . what i am trying to say is that GMAT questions always are always unambiguous such that even you do not read the official disclaimers (eg: GMAT always tests real numbers ) , you will be able to arrive a definitive conclusion.
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Re: What is the value of x? [#permalink] New post   03 May 2015, 11:52
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Lucky2783 wrote:
Bunuel wrote:
Shalabh09 wrote:
What is the value of x?

(1) \(x^2+x+10=16\)
(2) \(x=4y^4+2y^2+2\)

Could an Expert please help with the difficulty level as well? I'm not sure if this falls in 600-700.


What is the value of x?

(1) \(x^2+x+10=16\) --> \(x^2+x-6=0\) --> x=-3 or x=2. Not sufficient.

(2) \(x=4y^4+2y^2+2\). Without knowing the value of y we cannot get the value of x. Not sufficient.
Notice that from this statement it follows that x must be positive: \(x=4y^4+2y^2+2=(nonnegative)+(nonnegative)+(positive)=positive\).

(1)+(2) Since from (2) x is positive, then from (1) we are left with x=2. Sufficient.

Answer: C.

P.S. As for the difficulty level, I'd say it's ~550-600 level question.


Hi Bunuel

There is no information given about Y , Y can be a complex number too, so shouldn't i choose answer E ?

The roots of the polynomial \(4y^4+2y^2+5=0\) are:
\(Y_1=0.6588+0.82705∗i\)
\(Y_2=0.6588−0.82705∗i\)
\(Y_3=−0.6588+0.82705∗i\)
\(Y_4=−0.6588−0.82705∗i\)

Generally i have seen that GMAT questions always give hints about the numbers, in this question nothing has been told about Y,Strange.

noticed that there are a few questions floating in gmatclub , testing knowledge on complex numbers...
eg : if-the-square-of-the-sum-of-the-roots-of-a-quadratic-equation-197205.html


All numbers used on the GMAT are real numbers by default.
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Re: What is the value of x? [#permalink] New post   04 May 2015, 13:53
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Hi Shalabh09,

With a Q41 or Q42, your "issue(s)" might not be with content knowledge but in how you "see" (and respond to) the GMAT. It might be that you actually need to focus more on tactics, pattern-matching, etc. and less on doing more of the same things that you've already done.

1) How long have you studied?
2) What practice resources have you used so far?
3) What is your goal score?

GMAT assassins aren't born, they're made,
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