January 22, 2019 January 22, 2019 10:00 PM PST 11:00 PM PST In case you didn’t notice, we recently held the 1st ever GMAT game show and it was awesome! See who won a full GMAT course, and register to the next one. January 26, 2019 January 26, 2019 07:00 AM PST 09:00 AM PST Attend this webinar to learn how to leverage Meaning and Logic to solve the most challenging Sentence Correction Questions.
Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 14 Apr 2010
Posts: 179

Is the product of x and y a prime number?
[#permalink]
Show Tags
12 Aug 2010, 22:11
Question Stats:
61% (01:11) correct 39% (01:04) wrong based on 178 sessions
HideShow timer Statistics
Is the product of x and y a prime number? (1) x^2 = 1 (2) y is positive and prime; x is positive but not prime (C) 2008 GMAT Club  m01#30
Official Answer and Stats are available only to registered users. Register/ Login.



Intern
Joined: 30 May 2010
Posts: 1
Location: Hyderabad, India

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
12 Aug 2010, 23:10
A) \(x^2\)=1 => x = 1 or 1, y can be anything. Let y=4 => xy= 4 or 4 which is not a prime B) Let y=3 and x=9 => xy=27, which is not a prime.
C)x=1 and y is positive & prime => xy = y which is prime.
So answer should be C



Manager
Joined: 03 Jun 2010
Posts: 143
Location: United States (MI)
Concentration: Marketing, General Management
WE: Business Development (Consumer Products)

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
13 Aug 2010, 08:16
1) clearly unsuf 2) if x=1, y=2, xy prime, if x=6, y=2, xy not prime. Uns 1)+2) x=1 y is prime, so xy prime also (C) Am i right that eliminated "1" when consider (1)+(2)?



Intern
Joined: 10 Apr 2008
Posts: 21

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
13 Aug 2010, 09:59
Why isn't the answer B?
If Y is positive and prime, and X is positive and not prime then xy will always be a nonprime number, right?
What am I missing?



Current Student
Joined: 06 Oct 2009
Posts: 81
Location: Mexico
Concentration: Entrepreneurship, Finance
GPA: 3.85
WE: Sales (Commercial Banking)

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
13 Aug 2010, 10:51
I go with C
For a product of numbers to be prime, the other number has to be 1.
1) You can get positive or negative. Prime numbers are always positive. YES for 1 No for 1 Not Suff.
2) Tells us that Y is prime and that the other number is positive (it could be any number). Not Suff
1 + 2 = 1. YES



Intern
Joined: 12 Mar 2010
Posts: 9

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
13 Aug 2010, 23:47
C :
1) x^2 = 1 => x can be 1 or 1 and it doesn't tell us anything about y, hence insufficient 2) y>0 and y prime, x>0 and x prime Gives two possibilities: x=1 (1 is not a prime number) & y=3 => xy = 3 is prime or x=4 & y=3 => xy = 12 which is not prime
Together: from 1) we know that x= 1 or 1 and from 2) (x>0) we are sure that it is 1 And if y is prime, x=1 then xy=y and the result is prime



Manager
Joined: 14 Apr 2010
Posts: 179

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
14 Aug 2010, 06:57
OA = C While combining the two statements, I didn't pay attention to "x is positive" part.. such a stupid mistake



Director
Status: Apply  Last Chance
Affiliations: IIT, Purdue, PhD, TauBetaPi
Joined: 17 Jul 2010
Posts: 621
Schools: Wharton, Sloan, Chicago, Haas
WE 1: 8 years in Oil&Gas

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
14 Aug 2010, 07:28
I don't understand. 1. x = 1 or 1, I get the insuf part 2. y=prime, x=not prime, right here xy we know is NOT prime. Why cannot I stop at B? I don't need the A part anymore. If I look at C, then ok x=1, xy=y, y is prime, ok xy is prime. But my question is why do I have to go to C.. Is it not that if I can answer with less information, I go with that option? I also hear that the options cannot contradict each other, which means that x=1 and hence I am forced to use that information to go with C? Bunuel please enlighten...
_________________
Consider kudos, they are good for health



Math Expert
Joined: 02 Sep 2009
Posts: 52385

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
14 Aug 2010, 08:51
bibha wrote: Is the product of x and y a prime number? 1)x^2 = 1 2) y is positive and prime; x is positive but not prime (C) 2008 GMAT Club  m01#30
I have a small confusion with this.. Will tell you after you get the answer. Couple of things: 1. Only positive integers can be prime numbers; 2. 1 is not prime number. Next, product of two integers \(x\) and \(y\) to be prime: either one of them must be \(1\) and another \(prime\), for example (1,3), (1,7)... OR one of them must be \(1\) and another \(prime\), for example (1,7), (1,13) .... Back to the original question:Is the product of x and y a prime number?Question: is \(xy=prime\)? (1) \(x^2=1\) > \(x=1\) or \(x=1\). Not sufficient. (2) \(y=prime\); \(x>0\neq{prime}\) > if \(y=prime\) and \(x=1\) then answer would be YES but if \(y=prime\) and \(x=4\) (or any other nonprime integer >1 or some fractions) then answer would be NO. Not sufficient. (1)+(2) as from (1) \(x=1\) or \(x=1\) and from (2) \(x>0\) then \(x=1\), so \(xy=1*prime=prime\). Sufficient. Answer: C. Hope it's clear.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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 DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Director
Status: Apply  Last Chance
Affiliations: IIT, Purdue, PhD, TauBetaPi
Joined: 17 Jul 2010
Posts: 621
Schools: Wharton, Sloan, Chicago, Haas
WE 1: 8 years in Oil&Gas

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
14 Aug 2010, 09:03
Excellent Bunuel. it is the x=1 case that does the complication. Thanks for all your time and help!
_________________
Consider kudos, they are good for health



Manager
Joined: 07 Feb 2010
Posts: 129

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
15 Aug 2010, 17:54
Is the product of x and y a prime number? 1)x^2 = 1 2) y is positive and prime; x is positive but not prime
stat 1: no y value, not sufficient stat 2: depends on the value of x, for ex if x=1 xy is prime number, if x=2 xy is not prime from stat 1 and 2 x=1 so answer is c



Senior Manager
Joined: 29 Jan 2011
Posts: 283

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
24 Sep 2011, 20:59
Bunuel wrote: bibha wrote: Is the product of x and y a prime number? 1)x^2 = 1 2) y is positive and prime; x is positive but not prime (C) 2008 GMAT Club  m01#30
I have a small confusion with this.. Will tell you after you get the answer. Couple of things: 1. Only positive integers can be prime numbers; 2. 1 is not prime number. Next, product of two integers \(x\) and \(y\) to be prime: either one of them must be \(1\) and another \(prime\), for example (1,3), (1,7)... OR one of them must be \(1\) and another \(prime\), for example (1,7), (1,13) .... Back to the original question:Is the product of x and y a prime number? Question: is \(xy=prime\)? (1) \(x^2=1\) > \(x=1\) or \(x=1\). Not sufficient. (2) \(y=prime\); \(x>0\neq{prime}\) > if \(y=prime\) and \(x=1\) then answer would be YES but if \(y=prime\) and \(x=4\) (or any other nonprime integer >1 or some fractions) then answer would be NO. Not sufficient. (1)+(2) as from (1) \(x=1\) or \(x=1\) and from (2) \(x>0\) then \(x=1\), so \(xy=1*prime=prime\). Sufficient. Answer: C. Hope it's clear. Hi Bunnel, In the second statement example... We cant take x = 2 since it is given x is positive and not prime but 2 is prime...



Manager
Status: Target MBA
Joined: 20 Jul 2010
Posts: 134
Location: Singapore

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
25 Sep 2011, 03:04
bibha wrote: Is the product of x and y a prime number? 1)x^2 = 1 2) y is positive and prime; x is positive but not prime (C) 2008 GMAT Club  m01#30
I have a small confusion with this.. Will tell you after you get the answer. 1) x^2 = 1 means x=1 or x=1 y can be any number So x*y may or may not be prime. Hence we can eliminate A and D 2) y = +ve and prime, so y = 2,3,5... x = +ve and not nonprime, so x=1,4,6... So, x*y = (2, 3, 6, 12...) may or may not be prime. Hence we can eliminate B 3) Combining both x can only be 1 and y can only be a prime number Hence x*y = prime. We can eliminate E. So answer should be C.
_________________
Thanks and Regards, GM.



CEO
Joined: 11 Sep 2015
Posts: 3355
Location: Canada

Re: Is the product of x and y a prime number?
[#permalink]
Show Tags
23 Apr 2018, 05:46
bibha wrote: Is the product of x and y a prime number?
(1) x² = 1 (2) y is positive and prime; x is positive but not prime
Target question: Is the product xy a prime number? Statement 1: x² = 1 This tells us that either x = 1 or x = 1, but it provides NO INFORMATION about y. Consider these two possible cases: Case a: x = 1 and y = 3. In this case, xy = (1)(3) = 3, which is prime. So, the answer to the target question is YES, the product xy IS primeCase b: x = 1 and y = 6. In this case, xy = (1)(6) = 6, which is NOT prime. So, the answer to the target question is NO, the product xy is NOT primeSince we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT Statement 2: y is positive and prime; x is positive but not primeLet's test some possible cases again... Case a: x = 1 (aside: 1 is NOT prime) and y = 3. In this case, xy = (1)(3) = 3, which is prime. So, the answer to the target question is YES, the product xy IS primeCase b: x = 4 and y = 3. In this case, xy = (4)(3) = 12, which is NOT prime. So, the answer to the target question is NO, the product xy is NOT primeSince we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT Statements 1 and 2 combined Statement 1 tells us that either x = 1 or x = 1 Statement 2 tells us that x is POSITIVE, so we now know that x MUST equal 1If x = 1, then the product xy = (1)(y) = y Statement 2 also tells us that y is PRIME, and we just concluded that the product xy = yThis means the product xy MUST BE PRIMESince we can answer the target question with certainty, the combined statements are SUFFICIENT Answer: C Cheers, Brent
_________________
Test confidently with gmatprepnow.com




Re: Is the product of x and y a prime number? &nbs
[#permalink]
23 Apr 2018, 05:46






