GMAT Question of the Day - Daily to your Mailbox; hard ones only

It is currently 22 Oct 2019, 09:21

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Close

Request Expert Reply

Confirm Cancel

Is ab a prime number?

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Find Similar Topics 
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 58422
Is ab a prime number?  [#permalink]

Show Tags

New post 07 Oct 2015, 06:34
1
6
00:00
A
B
C
D
E

Difficulty:

  45% (medium)

Question Stats:

57% (01:02) correct 43% (00:59) wrong based on 324 sessions

HideShow timer Statistics

CEO
CEO
User avatar
D
Status: GMATINSIGHT Tutor
Joined: 08 Jul 2010
Posts: 2978
Location: India
GMAT: INSIGHT
Schools: Darden '21
WE: Education (Education)
Reviews Badge
Re: Is ab a prime number?  [#permalink]

Show Tags

New post 07 Oct 2015, 06:43
Bunuel wrote:
Is ab a prime number?

(1) a is a prime number.
(2) b is not a prime number.


Kudos for a correct solution.


Question : Is ab a prime number?

Statement 1: a is a prime number.
if b is 1 then ab will be a prime number. But
If b is not equal to 1 then ab will not be a Prime Number. Hence,
NOT SUFFICIENT


Statement 2: b is not a prime number.
if b is 1 and a is prime then ab will be a prime number. But
If b is equal to 4 and a is any Integer then ab will not be a Prime Number. Hence,
NOT SUFFICIENT

Combining the two statements
Case 1: a = 2, b=1, ab=2 i.e. PRIME NUMBER
Case 2: a = 2, b=4, ab=8 i.e. NOT A PRIME NUMBER. Hence,
NOT SUFFICIENT

Answer: option E
_________________
Prosper!!!
GMATinsight
Bhoopendra Singh and Dr.Sushma Jha
e-mail: info@GMATinsight.com I Call us : +91-9999687183 / 9891333772
Online One-on-One Skype based classes and Classroom Coaching in South and West Delhi
http://www.GMATinsight.com/testimonials.html

ACCESS FREE GMAT TESTS HERE:22 ONLINE FREE (FULL LENGTH) GMAT CAT (PRACTICE TESTS) LINK COLLECTION
GMAT Club Legend
GMAT Club Legend
User avatar
V
Joined: 12 Sep 2015
Posts: 4017
Location: Canada
Re: Is ab a prime number?  [#permalink]

Show Tags

New post 07 Oct 2015, 08:06
1
Bunuel wrote:
Is ab a prime number?

(1) a is a prime number.
(2) b is not a prime number.


Kudos for a correct solution.


Target question: Is ab a prime number?

Statement 1: a is a prime number
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several scenarios that satisfy statement 1. Here are two:
Case a: a = 2 and b = 1 , in which case ab = 2, and 2 IS prime
Case b: a = 2 and b = 6 , in which case ab = 12, and 12 in NOT prime
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Aside: For more on this idea of plugging in values when a statement doesn't feel sufficient, you can read my article: http://www.gmatprepnow.com/articles/dat ... lug-values

Statement 2: b is not a prime number
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several scenarios that satisfy statement 2. Here are two:
Case a: a = 2 and b = 1 , in which case ab = 2, and 2 IS prime
Case b: a = 2 and b = 6 , in which case ab = 12, and 12 in NOT prime
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
There are still several possible scenarios that meet both statements. Here are two:
Case a: a = 2 and b = 1 , in which case ab = 2, and 2 IS prime
Case b: a = 2 and b = 6 , in which case ab = 12, and 12 in NOT prime

Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer = E

Cheers,
Brent
_________________
Test confidently with gmatprepnow.com
Image
Manager
Manager
User avatar
Joined: 21 Jun 2014
Posts: 98
Location: United States
Concentration: General Management, Strategy
GMAT 1: 630 Q45 V31
GPA: 3.4
WE: Engineering (Computer Software)
Re: Is ab a prime number?  [#permalink]

Show Tags

New post 07 Oct 2015, 10:35
From Option A:
if b=1 then ab is a prime number else ab is not a prime number .Not sufficient
From Option B:
if b=1 and a is a prime number then ab is a prime number .For other values of a ,ab is not a prime number.
else if b=2,3,4... ab is not a prime number .Not sufficient.

Correct answer is E.
_________________
Regards,
Manish Khare
"Every thing is fine at the end. If it is not fine ,then it is not the end "
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 58422
Re: Is ab a prime number?  [#permalink]

Show Tags

New post 11 Oct 2015, 07:11
Bunuel wrote:
Is ab a prime number?

(1) a is a prime number.
(2) b is not a prime number.


Kudos for a correct solution.


VERITAS PREP OFFICIAL SOLUTION:

Question Type: Yes/No. This question stem asks: “Is ab a prime number?”

Given information in the question stem or diagram: There is no important information given in the question stem.

Statement 1: a is a prime number. This statement is not sufficient but it is tricky. Using conceptual knowledge, many people will assume that the product ab could never be prime. When you multiply any prime number with another positive integer, it could never be prime unless that integer is 1. However, since b could be 1 or any fraction, this statement is not sufficient, and you can eliminate choices A and D. For instance, if a is 7 and b is 1 or 2 7 then the answer is “yes.” However if b is any positive integer other than 1, then the answer is “no.” Note: This question is about avoiding assumptions! Do not assume that b is a whole number, and don’t forget about the very important number 1.

Statement 2: b is not a prime number. If b is not prime then b could easily be a fraction or equal to 1. In either case it is very possible for ab to be a prime number and it is very easy to show that it might not be. Not sufficient. Eliminate answer B.

Together: Taken together you know that a is prime and b is not. But this leaves you in the same dilemma as with Statement 1 alone. Certainly a and b can multiply to equal a non-prime number such as “a = 2 and b = 6.” 12 is not a prime number. However, if b =1 then whatever prime number a equals will remain and ab will be that prime number. As in the first statement, the key in this problem is to avoid assumptions and remember that nothing limits b to an integer and nothing excludes the value of 1. Answer is E.
_________________
Intern
Intern
avatar
Joined: 20 Dec 2018
Posts: 45
Re: Is ab a prime number?  [#permalink]

Show Tags

New post 20 Dec 2018, 03:37
Statement 1. A is a prime number.
Case 1. If ‘b’ = 1; ab is a prime number.
Case 2. If ‘b’ not equal to 1; ab is not a prime number. Hence, Insufficient.
Statement 2. ‘b’ is not a prime number.
Case 1. Let b = 4(non-prime) and a = 1/2. Then, ab = 2 i.e. a prime number.
Case 2. Let b = 4(non-prime) and a = 1. Then ab = 4 i.e. a composite number.Hence, Insufficient.
Statement 1 & 2 together. ‘a’ is a prime and ‘b’ is a non- prime number.
Case 1. Let a = 2(Prime number) and b = 1 (non-prime number). Then, ab = 2 i.e. a prime number.
Case 2. Let a = 2(Prime number) and b = 4 (non- prime number). Then, ab= 8 i.e. non-prime number. Hence, Insufficient.
Hence, e is the answer.
GMAT Club Bot
Re: Is ab a prime number?   [#permalink] 20 Dec 2018, 03:37
Display posts from previous: Sort by

Is ab a prime number?

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  





Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne