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k is a positive integer. Is k prime?

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k is a positive integer. Is k prime? [#permalink] New post 30 Apr 2013, 02:54
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k is a positive integer. Is k prime?

(1) At least one number in the set {1, k, k + 7} is prime.
(2) k is odd.
[Reveal] Spoiler: OA

Last edited by Bunuel on 30 Apr 2013, 02:57, edited 1 time in total.
RENAMED THE TOPIC.
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Re: k is a positive integer. Is k prime? [#permalink] New post 30 Apr 2013, 03:01
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k is a positive integer. Is k prime?

Notice that we are told that k is a positive integer.

(1) At least one number in the set {1, k, k + 7} is prime. Clearly insufficient.
(2) k is odd. Clearly insufficient.

(1)+(2) The set is {1, k=odd, k+7=even>2}. Since at least on number in this set is a prime, then it must be k. Sufficient.

Answer: C.
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Kudos [?]: 3 [0], given: 15

Re: k is a positive integer. Is k prime? [#permalink] New post 30 Apr 2013, 03:36
Bunuel wrote:
k is a positive integer. Is k prime?

Notice that we are told that k is a positive integer.

(1) At least one number in the set {1, k, k + 7} is prime. Clearly insufficient.
(2) k is odd. Clearly insufficient.

(1)+(2) The set is {1, k=odd, k+7=even>2}. Since at least on number in this set is a prime, then it must be k. Sufficient.

Answer: C.


you made it look so simple...thanks a ton Bunuel.
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Re: k is a positive integer. Is k prime? [#permalink] New post 14 Jan 2014, 06:38
Bunuel wrote:
k is a positive integer. Is k prime?

Notice that we are told that k is a positive integer.

(1) At least one number in the set {1, k, k + 7} is prime. Clearly insufficient.
(2) k is odd. Clearly insufficient.

(1)+(2) The set is {1, k=odd, k+7=even>2}. Since at least on number in this set is a prime, then it must be k. Sufficient.

Answer: C.


I thought it was A because I thought that in a set, the numbers are organized by size. So I assumed that k is prime because any prime + 7 = no prime. But obviously a set is not sorted. I get the rest of the reasoning. Am I right assuming that sets don't need to be sorted? Please verify!

Thanks!
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Re: k is a positive integer. Is k prime? [#permalink] New post 14 Jan 2014, 06:57
Expert's post
unceldolan wrote:
Bunuel wrote:
k is a positive integer. Is k prime?

Notice that we are told that k is a positive integer.

(1) At least one number in the set {1, k, k + 7} is prime. Clearly insufficient.
(2) k is odd. Clearly insufficient.

(1)+(2) The set is {1, k=odd, k+7=even>2}. Since at least on number in this set is a prime, then it must be k. Sufficient.

Answer: C.


I thought it was A because I thought that in a set, the numbers are organized by size. So I assumed that k is prime because any prime + 7 = no prime. But obviously a set is not sorted. I get the rest of the reasoning. Am I right assuming that sets don't need to be sorted? Please verify!

Thanks!


A set is not necessarily ordered but this has nothing to do with the first statement.

The question asks whether k is a prime number. (1) says that at least one number in the set {1, k, k + 7} is prime. Sure k can be prime but k can as well be for example 4, so not a prime and in this case k+7=11 is a prime.

Hope it's clear.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

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?
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Re: k is a positive integer. Is k prime?   [#permalink] 14 Jan 2014, 06:57
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