Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 06 Jun 2014
Posts: 91
Location: United States
Concentration: Finance, General Management
GPA: 3.47

If a and b are nonnegative integers, is ab prime? [#permalink]
Show Tags
05 Feb 2016, 18:10
Question Stats:
48% (00:25) correct 53% (00:18) wrong based on 240 sessions
HideShow timer Statistics
If a and b are nonnegative integers, is ab prime? 1) a = 17 2) b = 91
Official Answer and Stats are available only to registered users. Register/ Login.



SVP
Joined: 06 Nov 2014
Posts: 1888

Re: If a and b are nonnegative integers, is ab prime? [#permalink]
Show Tags
05 Feb 2016, 21:20
zxcvbnmas wrote: If a and b are nonnegative integers, is ab prime?
1) a = 17 2) b = 91 Statement 1: This tells us only about a. If b were 1, then product ab = 17 and it would be prime. If b were 2 (or any other number), then product ab = 34 and it would not be prime. Hence insufficient Statement 2: This tells us only about b But it says that b = 91 and this is not a prime. So if we multiply a non prime number with any other number, we will get a non prime number. Hence product ab is not prime. Hence Sufficient Option B



Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 5600
GPA: 3.82

Re: If a and b are nonnegative integers, is ab prime? [#permalink]
Show Tags
06 Feb 2016, 19:34
Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution. If a and b are nonnegative integers, is ab prime? 1) a = 17 2) b = 91 In the original condition, there are 2 variables, which should match with the number of equations. So, you need 2 equations. However, for 1) 1 equation, for 2) 1 equation, which is likely to make C the answer. When 1) & 2), ab=17*91=/ prime, which is no and sufficient. Therefore, the answer is C. However, since this is an integer question, one of the key questions, apply the mistake type 4(A). For 1), a=17, b=1 > yes, a=17, b=18 > no, which is not sufficient. For 2), b=91=7*13 > ab=/ prime, which is always no and sufficient. Therefore, the answer is B. > For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The oneandonly World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only $99 for 3 month Online Course" "Free Resources30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons  try it yourself"



Intern
Joined: 06 Aug 2015
Posts: 3

Re: If a and b are nonnegative integers, is ab prime? [#permalink]
Show Tags
16 Feb 2016, 16:19
Hi. I've just solved this problem, from a Veritas Prep test. My question is, given the nonnegative phrase, can't a or b be zero? In that case, S1 a = 17*b=0 = 0, not prime. S2 a=0 * b=91 equals zero, not prime. That's why I think both statements are necessary (C). Appreciate comments. Thanks!



Math Expert
Joined: 02 Aug 2009
Posts: 5938

Re: If a and b are nonnegative integers, is ab prime? [#permalink]
Show Tags
16 Feb 2016, 18:26
nramirez wrote: Hi. I've just solved this problem, from a Veritas Prep test. My question is, given the nonnegative phrase, can't a or b be zero? In that case, S1 a = 17*b=0 = 0, not prime. S2 a=0 * b=91 equals zero, not prime. That's why I think both statements are necessary (C). Appreciate comments. Thanks! Hi, you are correct in your assumption that one of the numbers can be 0 also, but it will still not make a difference to the answer.. Statement 2 tells us that ab can not be prime, since b=91, and a can be prime,2 , or 0 or 9 etc the product will never be PRIME suff
_________________
Absolute modulus :http://gmatclub.com/forum/absolutemodulusabetterunderstanding210849.html#p1622372 Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
GMAT online Tutor



NonHuman User
Joined: 09 Sep 2013
Posts: 7057

Re: If a and b are nonnegative integers, is ab prime? [#permalink]
Show Tags
08 Mar 2017, 23:07
Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up  doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources



Intern
Joined: 23 Aug 2010
Posts: 5

Zero is a nonnegative integer. Data Sufficiency Question on Veritas [#permalink]
Show Tags
27 May 2017, 22:24
I came across this Data Sufficiency question on one of the Veritas Tests. If a and b are nonnegative integers, is ab prime? a = 17 b = 91 The solution appears to be B, according to Veritas. Because b=91, it is obvious that anything multiplied with a nonprime number is a nonprime number. But what about 0? Zero is a nonnegative integer. It's not sufficient to know that b=91 because if a=0, then ab is neither prime nor composite. I selected E as my answer which is incorrect, according to the solution key. Please clarify! Thanks!



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 8102
Location: Pune, India

Re: Zero is a nonnegative integer. Data Sufficiency Question on Veritas [#permalink]
Show Tags
27 May 2017, 22:54
priyer111 wrote: I came across this Data Sufficiency question on one of the Veritas Tests. If a and b are nonnegative integers, is ab prime? a = 17 b = 91 The solution appears to be B, according to Veritas. Because b=91, it is obvious that anything multiplied with a nonprime number is a nonprime number. But what about 0? Zero is a nonnegative integer. It's not sufficient to know that b=91 because if a=0, then ab is neither prime nor composite. I selected E as my answer which is incorrect, according to the solution key. Please clarify! Thanks! Yes, a can be 0. In that case, using statement 2 alone, we get that ab = 0*91 = 0. 0 is not prime. So if the question is: Is ab prime? You answer "No." If a has any other non negative integer value, ab = 91a and will not be prime. Again, you answer "No" to the question. So in every case, your answer will be "No." and hence statement 2 is sufficient. Note that we don't care whether ab is 0 or a composite number. The point is that it will not be prime.
_________________
Karishma Veritas Prep  GMAT Instructor My Blog
Get started with Veritas Prep GMAT On Demand for $199
Veritas Prep Reviews




Re: Zero is a nonnegative integer. Data Sufficiency Question on Veritas
[#permalink]
27 May 2017, 22:54






