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GMAT Diagnostic Test Question 4

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GMAT Diagnostic Test Question 4 [#permalink] New post 06 Jun 2009, 20:06
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GMAT Diagnostic Test Question 4
Field: Arithmetic, absolute values
Difficulty: 600


Is x a prime number?

(1) |x|=2
(2) x^2=4
[Reveal] Spoiler: OA

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Last edited by Bunuel on 06 Oct 2013, 23:04, edited 2 times in total.
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 12 Jun 2009, 06:27
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Explanation:
Official Answer: E

(1) |x|=2 --> x=2 or x=-2. Not sufficient.
(2) x^2=4 --> the same solution: x=2 or x=-2. Not sufficient.

(1)+(2) Nothing new: either x=2=prime or x=-2\neq{prime}. Not sufficient.

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Last edited by bb on 28 Sep 2013, 10:40, edited 1 time in total.
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 10 Aug 2009, 18:24
Prime number is a number that can be divided only by itself and the number one. For example, three and seven are prime numbers.
So I think the correct answer is "E".
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 16 Oct 2009, 04:35
why we can't have negative prime numbers? I think we do have -ve prime numbers..
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 16 Oct 2009, 08:57
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I'm afraid GMAC won't agree with you. Prime numbers can't be negative.
KocharRohit wrote:
why we can't have negative prime numbers? I think we do have -ve prime numbers..

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 08 Nov 2009, 09:46
Statement (2) - Should'nt we consider only the positive value for square root of 4 (and not -2)?

If that is true Stmt 2 would be sufficient, Am I missing something here?
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 09 Nov 2009, 03:21
Is X a prime integer?

Statement (1): There are two possible values, +2 & -2
Statement (2): there are 2 possible values +2, -2

Prime number is a number that can be divided only by itself and the number one. For example, three and seven are prime numbers.
So , shouldn't the answer be D cause we get the answer anyway that X is not a prime integer since both the answers are +2 & -2
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 09 Nov 2009, 03:24
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This is one of the common misunderstandings. If the question asked for the value of \sqrt{4}, then it would surely be 2 and not -2 on GMAT. However, when it is stated that X^2=4, you have two options, 2 and -2. There's nothing wrong in squaring a negative number.
Does this explanation make any sense?
kl wrote:
Statement (2) - Should'nt we consider only the positive value for square root of 4 (and not -2)?

If that is true Stmt 2 would be sufficient, Am I missing something here?

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 09 Nov 2009, 03:27
+2 is a prime, -2 is not. Combining the two statements still doesn't give us more information. We can't be certain if X is prime (2) or not (-2) as both values of X are possible.
pdew wrote:
Is X a prime integer?

Statement (1): There are two possible values, +2 & -2
Statement (2): there are 2 possible values +2, -2

Prime number is a number that can be divided only by itself and the number one. For example, three and seven are prime numbers.
So , shouldn't the answer be D cause we get the answer anyway that X is not a prime integer since both the answers are +2 & -2

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 26 Nov 2009, 07:09
seemed so simple...thanks for the question and the lesson learned
story for the water cooler next week: "What'd ya do Thanksgiving?" response: "Learned that -2 is not prime." :-D

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 24 Jan 2010, 09:06
This question tricked me.
Actually i though |X| is only positive integer.
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 24 Jan 2010, 09:24
Expert's post
beautifullLif3 wrote:
This question tricked me.
Actually i though |X| is only positive integer.


Absolute value, |x| in our case, is never negative, |some expression|>=0. But the expression IN it (in ||) can be negative. So |x| >=0, but x can take ANY value, positive, zero, negative.

We are asked whether x is a prime number not absolute value of x (not |x|).

One more thing regarding this question: prime numbers are ONLY positive. 3 is prime, while -3 is not.

Hope it helps.

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 16 Feb 2010, 06:45
Bunuel wrote:
beautifullLif3 wrote:
This question tricked me.
Actually i though |X| is only positive integer.


Absolute value, |x| in our case, is never negative, |some expression|>=0. But the expression IN it (in ||) can be negative. So |x| >=0, but x can take ANY value, positive, zero, negative.

We are asked whether x is a prime number not absolute value of x (not |x|).

One more thing regarding this question: prime numbers are ONLY positive. 3 is prime, while -3 is not.

Hope it helps.



since we're on the subject of prime numbers, one more addition to this would be that prime numbers are always odd.
thus it becomes positive odd numbers, which can be divided by itself and 1

is it right that we go about handing absolute equations like this:
for e.g. if we have |(any expression)| = 4
then that expression could mean either of these:
(any expression) = 4 OR (any expression) = -4

we take one positive value of the number on the right hand side and one negative value?

is my understanding correct about absolute values?
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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 16 Feb 2010, 07:03
Expert's post
meganbaxter1 wrote:
Bunuel wrote:
beautifullLif3 wrote:
This question tricked me.
Actually i though |X| is only positive integer.


Absolute value, |x| in our case, is never negative, |some expression|>=0. But the expression IN it (in ||) can be negative. So |x| >=0, but x can take ANY value, positive, zero, negative.

We are asked whether x is a prime number not absolute value of x (not |x|).

One more thing regarding this question: prime numbers are ONLY positive. 3 is prime, while -3 is not.

Hope it helps.



since we're on the subject of prime numbers, one more addition to this would be that prime numbers are always odd.
thus it becomes positive odd numbers, which can be divided by itself and 1

is it right that we go about handing absolute equations like this:
for e.g. if we have |(any expression)| = 4
then that expression could mean either of these:
(any expression) = 4 OR (any expression) = -4

we take one positive value of the number on the right hand side and one negative value?

is my understanding correct about absolute values?


Red part is not correct: 2 is even and is prime, it's also the only even prime and the smallest prime.

About the absolute value you are correct.

I'd suggest to check "Absolute value" and "Number theory" chapters in Math Book and also the topic "Inequalities" (links in my signature).

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 02 Apr 2010, 04:15
Missed this because I assumed (wrongly) that -x can be a prime number.
However, can we also have -x! -negative factorials.
for example: (-4)! is it -4*-3.....*2*1?

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 02 Apr 2010, 04:40
Factorials of negative numbers are undefined on the GMAT. You won't encounter a negative number followed by a factorial sign on the GMAT.
gmatbull wrote:
Missed this because I assumed (wrongly) that -x can be a prime number.
However, can we also have -x! -negative factorials.
for example: (-4)! is it -4*-3.....*2*1?

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 02 Apr 2010, 04:56
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dzyubam wrote:
Factorials of negative numbers are undefined on the GMAT. You won't encounter a negative number followed by a factorial sign on the GMAT.
gmatbull wrote:
Missed this because I assumed (wrongly) that -x can be a prime number.
However, can we also have -x! -negative factorials.
for example: (-4)! is it -4*-3.....*2*1?

It was such a dummy question anyway.
from the basic explanation:
2! = 3!/3 = 2
1! = 2!/2 = 1
0! = 1!/1 = 1

-1! = 0!/0 => undefined
similarly, for all negative numbers:
(-x)! is undefined.

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 06 Apr 2010, 05:41
If we take x = 3 or 4 then it satisfies both stmt 1 and stmt 2 but we still dont know whether x is prime or not. because 3 is prime but 4 is not

Am i missing something :?:

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 08 Apr 2010, 02:00
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Bunuel wrote:
beautifullLif3 wrote:
This question tricked me.
Actually i though |X| is only positive integer.


Absolute value, |x| in our case, is never negative, |some expression|>=0. But the expression IN it (in ||) can be negative. So |x| >=0, but x can take ANY value, positive, zero, negative.

We are asked whether x is a prime number not absolute value of x (not |x|).

One more thing regarding this question: prime numbers are ONLY positive. 3 is prime, while -3 is not.

Hope it helps.



Bunuel Please clear my doubt here

Stmt 1 say that x can be 0,1,2,3,4
stmt 2 says -3>= x =>3
Both insufficient to make conclusion

Stmt 1 +Stmt 2 = x can be 3 or 4. Hence not sufficient
Please explain if i am wrong or missing something here?????

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Re: GMAT Diagnostic Test Question 4 [#permalink] New post 08 Apr 2010, 02:20
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Is x a prime integer?

(1) x! is not divisible by 5 --> x=0, 1, 2, 3, or 4. Not sufficient.
(2) |x|! is divisible by 6 --> |x|>2. Not sufficient.

(1)+(2) x=3 (prime) or x=4 (not prime). Not sufficient.

Answer: E (Just noticed that OA is C, which I think is not correct).

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COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

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Re: GMAT Diagnostic Test Question 4   [#permalink] 08 Apr 2010, 02:20
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