Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 22 Apr 2011
Posts: 164
Schools: Mccombs business school, Mays business school, Rotman Business School,

A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
15 May 2012, 21:48
Question Stats:
36% (01:30) correct 64% (01:46) wrong based on 410 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
Official Answer and Stats are available only to registered users. Register/ Login.
_________________
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




Math Expert
Joined: 02 Sep 2009
Posts: 49206

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
16 May 2012, 00:38
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.
_________________
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




Manager
Joined: 05 Dec 2011
Posts: 79
Location: Canada
Concentration: Accounting, Finance
GMAT Date: 09082012
GPA: 3

Re: Sophie Germain
[#permalink]
Show Tags
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/thursdayswithron.cfm
Gmat Prep Questions: CR http://gmatclub.com/forum/gmatprepsc105446.html SC http://gmatclub.com/forum/gmatprepsc105446.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
Joined: 08 Apr 2012
Posts: 387

Re: Sophie Germain
[#permalink]
Show Tags
16 May 2012, 00:14
Hey geno Can you elabotare how you got the answer?



Director
Joined: 03 Feb 2013
Posts: 894
Location: India
Concentration: Operations, Strategy
GPA: 3.88
WE: Engineering (Computer Software)

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
11 Jan 2014, 09:06
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?
_________________
Thanks, Kinjal My Debrief : http://gmatclub.com/forum/hardworknevergetsunrewardedforever189267.html#p1449379 My Application Experience : http://gmatclub.com/forum/hardworknevergetsunrewardedforever18926740.html#p1516961 Linkedin : https://www.linkedin.com/in/kinjaldas/
Please click on Kudos, if you think the post is helpful



Math Expert
Joined: 02 Sep 2009
Posts: 49206

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
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.
_________________
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
Joined: 03 Feb 2013
Posts: 894
Location: India
Concentration: Operations, Strategy
GPA: 3.88
WE: Engineering (Computer Software)

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
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.
_________________
Thanks, Kinjal My Debrief : http://gmatclub.com/forum/hardworknevergetsunrewardedforever189267.html#p1449379 My Application Experience : http://gmatclub.com/forum/hardworknevergetsunrewardedforever18926740.html#p1516961 Linkedin : https://www.linkedin.com/in/kinjaldas/
Please click on Kudos, if you think the post is helpful



Math Expert
Joined: 02 Sep 2009
Posts: 49206

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
11 Jan 2014, 09:26
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.
_________________
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
Joined: 03 Feb 2013
Posts: 894
Location: India
Concentration: Operations, Strategy
GPA: 3.88
WE: Engineering (Computer Software)

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
11 Jan 2014, 10:34



Manager
Joined: 08 Jun 2015
Posts: 112

A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
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.



Current Student
Joined: 27 Jan 2013
Posts: 101
Location: India
GPA: 3.7

A "Sophie Germain" prime is
[#permalink]
Show Tags
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
Joined: 02 Sep 2009
Posts: 49206

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
22 Aug 2016, 12:51



Intern
Joined: 20 Sep 2011
Posts: 18
Concentration: Operations, International Business

A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
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
Joined: 20 Sep 2011
Posts: 18
Concentration: Operations, International Business

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
06 Sep 2016, 23:53
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



NonHuman User
Joined: 09 Sep 2013
Posts: 8062

Re: A “Sophie Germain” prime is any positive prime number p for
[#permalink]
Show Tags
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 Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources




Re: A “Sophie Germain” prime is any positive prime number p for &nbs
[#permalink]
12 Sep 2018, 08:53






