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

It is currently 27 Jun 2019, 01:29

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

A “Sophie Germain” prime is any positive prime number p for

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

Hide Tags

 
Manager
Manager
avatar
Joined: 22 Apr 2011
Posts: 124
Schools: Mccombs business school, Mays business school, Rotman Business School,
A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 15 May 2012, 21:48
10
27
00:00
A
B
C
D
E

Difficulty:

  95% (hard)

Question Stats:

32% (02:36) correct 68% (02:30) wrong based on 477 sessions

HideShow timer Statistics


A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189

_________________
some people are successful, because they have been fortunate enough and some people earn success, because they have been determined.....

please press kudos if you like my post.... i am begging for kudos...lol
Most Helpful Expert Reply
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 55804
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 16 May 2012, 00:38
8
1
12
alchemist009 wrote:
A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189


A prime number greater than 5 can have only the following four units digits: 1, 3, 7, or 9.

If the units digit of p is 1 then the units digit of 2p+1 would be 3, which is a possible units digit for a prime. For example consider p=11=prime --> 2p+1=23=prime;

If the units digit of p is 3 then the units digit of 2p+1 would be 7, which is a possible units digit for a prime. For example consider p=23=prime --> 2p+1=47=prime;

If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime;

If the units digit of p is 9 then the units digit of 2p+1 would be 9, which is a possible units digit for a prime. For example consider p=29=prime --> 2p+1=59=prime.

The product of all the possible units digits of Sophie Germain primes greater than 5 is 1*3*9=27.

Answer: D.

Hope it's clear.
_________________
General Discussion
Manager
Manager
avatar
Joined: 05 Dec 2011
Posts: 75
Location: Canada
Concentration: Accounting, Finance
GMAT Date: 09-08-2012
GPA: 3
Reviews Badge
Re: Sophie Germain  [#permalink]

Show Tags

New post Updated on: 16 May 2012, 00:47
+1 D

1*3*9=27, Rest of the Digits cannot be prime.

even cannot be prime. 5 not prime and (7)*2+1=15 not prime.
_________________
Thanks = +1 Kudos

Study from reliable sources!!

Thursdays with Ron: http://www.manhattangmat.com/thursdays-with-ron.cfm

Gmat Prep Questions:
CR http://gmatclub.com/forum/gmatprepsc-105446.html
SC http://gmatclub.com/forum/gmatprepsc-105446.html

Originally posted by geno5 on 15 May 2012, 23:33.
Last edited by geno5 on 16 May 2012, 00:47, edited 1 time in total.
Senior Manager
Senior Manager
avatar
Joined: 08 Apr 2012
Posts: 344
Re: Sophie Germain  [#permalink]

Show Tags

New post 16 May 2012, 00:14
Hey geno
Can you elabotare how you got the answer?
Director
Director
User avatar
B
Joined: 03 Feb 2013
Posts: 835
Location: India
Concentration: Operations, Strategy
GMAT 1: 760 Q49 V44
GPA: 3.88
WE: Engineering (Computer Software)
Reviews Badge
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 11 Jan 2014, 09:06
1
1
Bunuel wrote:
alchemist009 wrote:
A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189


A prime number greater than 5 can have only the following four units digits: 1, 3, 7, or 9.

If the units digit of p is 1 then the units digit of 2p+1 would be 3, which is a possible units digit for a prime. For example consider p=11=prime --> 2p+1=23=prime;

If the units digit of p is 3 then the units digit of 2p+1 would be 7, which is a possible units digit for a prime. For example consider p=23=prime --> 2p+1=47=prime;

If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime;

If the units digit of p is 9 then the units digit of 2p+1 would be 9, which is a possible units digit for a prime. For example consider p=29=prime --> 2p+1=59=prime.

The product of all the possible units digits of Sophie Germain primes greater than 5 is 1*3*9=27.

Answer: D.

Hope it's clear.


Why 7 is not considered for the final answer?
_________________
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 55804
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 11 Jan 2014, 09:09
kinjiGC wrote:
Bunuel wrote:
alchemist009 wrote:
A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189


A prime number greater than 5 can have only the following four units digits: 1, 3, 7, or 9.

If the units digit of p is 1 then the units digit of 2p+1 would be 3, which is a possible units digit for a prime. For example consider p=11=prime --> 2p+1=23=prime;

If the units digit of p is 3 then the units digit of 2p+1 would be 7, which is a possible units digit for a prime. For example consider p=23=prime --> 2p+1=47=prime;

If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime;

If the units digit of p is 9 then the units digit of 2p+1 would be 9, which is a possible units digit for a prime. For example consider p=29=prime --> 2p+1=59=prime.

The product of all the possible units digits of Sophie Germain primes greater than 5 is 1*3*9=27.

Answer: D.

Hope it's clear.


Why 7 is not considered for the final answer?


If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime greater than 5.
_________________
Director
Director
User avatar
B
Joined: 03 Feb 2013
Posts: 835
Location: India
Concentration: Operations, Strategy
GMAT 1: 760 Q49 V44
GPA: 3.88
WE: Engineering (Computer Software)
Reviews Badge
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 11 Jan 2014, 09:21
Bunuel wrote:
kinjiGC wrote:
Bunuel wrote:
A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189

A prime number greater than 5 can have only the following four units digits: 1, 3, 7, or 9.

If the units digit of p is 1 then the units digit of 2p+1 would be 3, which is a possible units digit for a prime. For example consider p=11=prime --> 2p+1=23=prime;

If the units digit of p is 3 then the units digit of 2p+1 would be 7, which is a possible units digit for a prime. For example consider p=23=prime --> 2p+1=47=prime;

If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime;

If the units digit of p is 9 then the units digit of 2p+1 would be 9, which is a possible units digit for a prime. For example consider p=29=prime --> 2p+1=59=prime.

The product of all the possible units digits of Sophie Germain primes greater than 5 is 1*3*9=27.

Answer: D.

Hope it's clear.


Why 7 is not considered for the final answer?


If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime greater than 5.


It might be simple, but I have a doubt here. The question asks product of all the possible unit digits of Sophie Germain primes.

As 47 is a sophie germain prime number and prime number and 47 is > than 5, so 7 being the unit digit should be included in the product to get the final answer. That is why I marked 189.
_________________
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 55804
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 11 Jan 2014, 09:26
2
kinjiGC wrote:
Bunuel wrote:
kinjiGC wrote:
Why 7 is not considered for the final answer?


If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime greater than 5.


It might be simple, but I have a doubt here. The question asks product of all the possible unit digits of Sophie Germain primes.

As 47 is a sophie germain prime number and prime number and 47 is > than 5, so 7 being the unit digit should be included in the product to get the final answer. That is why I marked 189.


A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. 47 is NOT a “Sophie Germain” prime because 2p+1=95, which is NOT a prime. Again, a “Sophie Germain” prime cannot have 7 as its units digit because the units digit of 2p+1 in this case would be 5. No prime greater than 5 has 5 as its units digit.

Hope it's clear.
_________________
Director
Director
User avatar
B
Joined: 03 Feb 2013
Posts: 835
Location: India
Concentration: Operations, Strategy
GMAT 1: 760 Q49 V44
GPA: 3.88
WE: Engineering (Computer Software)
Reviews Badge
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 11 Jan 2014, 10:34
Thanks Banuel.

I read the premise wrongly. :cry:
_________________
Manager
Manager
avatar
Joined: 08 Jun 2015
Posts: 102
A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 19 Jul 2015, 11:01
I see - I misunderstood the question at first...

It's asking for all of the possible UNIQUE units digits, not all of the units digits of SG primes multiplied out.

For example, 11, 23, 29, 53... are primes. Unique units are only 1, 3, and 9, which multiply to 27.

If interpreted literally, it would go on forever (i.e. answer = infinite).

Knowing this and the answer choices, there's only one intended Q&A combo possible - unique units and not infinite.

I do think, however, that they should have worded it more clearly as it is ambiguous.
Manager
Manager
User avatar
B
Joined: 27 Jan 2013
Posts: 97
Location: India
GMAT 1: 700 Q49 V35
GPA: 3.7
A "Sophie Germain" prime is  [#permalink]

Show Tags

New post 22 Aug 2016, 12:00
A "Sophie Germain" prime is any positive prime number p for which 2p+1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A: 37
B: 21
C: 27
D: 18
E: 9

IN URGENT NEED OF KUDOS !
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 55804
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 22 Aug 2016, 12:51
Intern
Intern
avatar
B
Joined: 20 Sep 2011
Posts: 18
Concentration: Operations, International Business
Schools: Ross '17, ISB '16, NUS '17
GMAT 1: 640 Q40 V35
GMAT ToolKit User Reviews Badge
A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 06 Sep 2016, 23:47
Bunuel wrote:
alchemist009 wrote:
A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189


A prime number greater than 5 can have only the following four units digits: 1, 3, 7, or 9.

If the units digit of p is 1 then the units digit of 2p+1 would be 3, which is a possible units digit for a prime. For example consider p=11=prime --> 2p+1=23=prime;

If the units digit of p is 3 then the units digit of 2p+1 would be 7, which is a possible units digit for a prime. For example consider p=23=prime --> 2p+1=47=prime;

If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime;

If the units digit of p is 9 then the units digit of 2p+1 would be 9, which is a possible units digit for a prime. For example consider p=29=prime --> 2p+1=59=prime.

The product of all the possible units digits of Sophie Germain primes greater than 5 is 1*3*9=27.

Answer: D.

Hope it's clear.


Need your help in understanding the question. Does not this question ask for product of "unit digits of Sophie prime numbers" and not unit digits of prime numbers.
In that case, the Sophie prime numbers greater than 5 are 7,11,23,47,59, .. which yields units digit as 1,3,7 and 9
Product would be 1 x 3 x 7x9 =189 Answer should be E.

Please help me understand.
Intern
Intern
avatar
B
Joined: 20 Sep 2011
Posts: 18
Concentration: Operations, International Business
Schools: Ross '17, ISB '16, NUS '17
GMAT 1: 640 Q40 V35
GMAT ToolKit User Reviews Badge
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 06 Sep 2016, 23:53
1
abani wrote:
Bunuel wrote:
alchemist009 wrote:
A “Sophie Germain” prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

A. 3
B. 7
C. 21
D. 27
E. 189


A prime number greater than 5 can have only the following four units digits: 1, 3, 7, or 9.

If the units digit of p is 1 then the units digit of 2p+1 would be 3, which is a possible units digit for a prime. For example consider p=11=prime --> 2p+1=23=prime;

If the units digit of p is 3 then the units digit of 2p+1 would be 7, which is a possible units digit for a prime. For example consider p=23=prime --> 2p+1=47=prime;

If the units digit of p is 7 then the units digit of 2p+1 would be 5, which is NOT a possible units digit for a prime;

If the units digit of p is 9 then the units digit of 2p+1 would be 9, which is a possible units digit for a prime. For example consider p=29=prime --> 2p+1=59=prime.

The product of all the possible units digits of Sophie Germain primes greater than 5 is 1*3*9=27.

Answer: D.

Hope it's clear.


Need your help in understanding the question. Does not this question ask for product of "unit digits of Sophie prime numbers" and not unit digits of prime numbers.
In that case, the Sophie prime numbers greater than 5 are 7,11,23,47,59, .. which yields units digit as 1,3,7 and 9
Product would be 1 x 3 x 7x9 =189 Answer should be E.

Please help me understand.


I went through the post and understood it. English is tough :oops:
Non-Human User
User avatar
Joined: 09 Sep 2013
Posts: 11448
Re: A “Sophie Germain” prime is any positive prime number p for  [#permalink]

Show Tags

New post 12 Sep 2018, 08:53
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 Club Bot
Re: A “Sophie Germain” prime is any positive prime number p for   [#permalink] 12 Sep 2018, 08:53
Display posts from previous: Sort by

A “Sophie Germain” prime is any positive prime number p for

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


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