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Which of the following are/is prime?

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Magoosh GMAT Instructor
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Which of the following are/is prime? [#permalink] New post 21 Feb 2013, 12:48
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A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

59% (01:36) correct 41% (00:35) wrong based on 99 sessions
Which of the following are/is prime?

I. 143
II. 147
III. 149

(A) II only
(B) III only
(C) I & II
(D) I & III
(E) I, II, & III


In order to handle questions like this without a calculator, it is very helpful to know
(a) divisibility tricks = http://magoosh.com/gmat/2012/gmat-divis ... shortcuts/
(b) factoring tricks = http://magoosh.com/gmat/2012/advanced-n ... -the-gmat/

Mike :-)
[Reveal] Spoiler: OA

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Mike McGarry
Magoosh Test Prep

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Kudos [?]: 52 [0], given: 48

Re: Which of the following are/is prime? [#permalink] New post 21 Feb 2013, 14:19
mikemcgarry wrote:
Which of the following are/is prime?

I. 143
II. 147
III. 149

(A) II only
(B) III only
(C) I & II
(D) I & III
(E) I, II, & III


In order to handle questions like this without a calculator, it is very helpful to know
(a) divisibility tricks = http://magoosh.com/gmat/2012/gmat-divis ... shortcuts/
(b) factoring tricks = http://magoosh.com/gmat/2012/advanced-n ... -the-gmat/

Mike :-)


Step 1, look over the three numbers quickly, realize 147 is divisible by 3. We know II is not prime.
Step 2, look at the answer choices. Realize the answer is (B) or (D). Therefore III must be prime.
Step 3, Verify if I is prime. Build a division table with other prime numbers and get your answer!
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Kudos [?]: 9 [0], given: 18

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Re: Which of the following are/is prime? [#permalink] New post 16 Mar 2013, 10:18
An interesting way to solve these kind of problems is to eliminate the obvious answers
1) 143 = 13*11 so we know it is not prime quickly eliminate all options having 1 as a choice
which leaves us with 2 ) and 3) as possible answers
From here on we find 147 = 3* 49 so we eliminate 147 which leaves us with 149 as the only possible solution
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Kudos [?]: 64 [0], given: 6

Re: Which of the following are/is prime? [#permalink] New post 16 Mar 2013, 14:13
Which of the following are/is prime?

I. 143:
- sum of digits multiple of 3? NO
- divisible by 7? NO
- divisible by 11? YES, 13x11=143 --> Not prime
II. 147
- sum of digits multiple of 3? YES, 1+4+7=12 --> Not prime

III. 149
- sum of digits multiple of 3? NO
- divisible by 7? NO
- divisible by 11? NO
- divisible by 13? NO

B is the only possible solution
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Re: Which of the following are/is prime?   [#permalink] 16 Mar 2013, 14:13
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