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# What is the value of prime number p ? (1) p+8 is prime.

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Senior SC Moderator
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What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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07 Feb 2017, 17:57
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Difficulty:

55% (hard)

Question Stats:

58% (01:33) correct 42% (01:34) wrong based on 108 sessions

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What is the value of prime number $$p$$?

(1) $$p+8$$ is prime.
(2) $$p−8$$ is prime.

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Re: What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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08 Feb 2017, 10:59
3
6
ziyuenlau wrote:
What is the value of prime number $$p$$?

(1) $$p+8$$ is prime.
(2) $$p−8$$ is prime.

Dear ziyuenlau,

I'm happy to respond.

This is a devilishly clever and difficult problem. Let's jump to the case in which we are considering both statements together.

Here are a few number sense rules.
1) In a set of three consecutive integers, one is always divisible by 3.
2) In a set of three evenly spaced integers, one is always divisible by 3.
(These rules generalize from 3 to n.)

The numbers (p - 8), p, and (p + 8) are three evenly spaced numbers, so it absolutely must be true that one of them is divisible by 3. The only way all three of them can be prime is if the one that is divisible by 3 is 3 itself! Thus:
p - 8 = 3
p = 11
p + 8 = 19
If p takes the value p = 11, this is the only way that all three numbers can be prime simultaneously. Thus, the value of p is unique determined. Combined the statements are sufficient. OA = (C).

Veritas wrote a truly beautiful problem here!

Does all this make sense?
Mike
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Re: What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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08 Feb 2017, 11:22
mikemcgarry wrote:
ziyuenlau wrote:
What is the value of prime number $$p$$?

(1) $$p+8$$ is prime.
(2) $$p−8$$ is prime.

Dear ziyuenlau,

I'm happy to respond.

This is a devilishly clever and difficult problem. Let's jump to the case in which we are considering both statements together.

Here are a few number sense rules.
1) In a set of three consecutive integers, one is always divisible by 3.
2) In a set of three evenly spaced integers, one is always divisible by 3.
(These rules generalize from 3 to n.)

The numbers (p - 8), p, and (p + 8) are three evenly spaced numbers, so it absolutely must be true that one of them is divisible by 3. The only way all three of them can be prime is if the one that is divisible by 3 is 3 itself! Thus:
p - 8 = 3
p = 11
p + 8 = 19
If p takes the value p = 11, this is the only way that all three numbers can be prime simultaneously. Thus, the value of p is unique determined. Combined the statements are sufficient. OA = (C).

Veritas wrote a truly beautiful problem here!

Does all this make sense?
Mike

This question might be inspired by the following two questions:
https://gmatclub.com/forum/is-the-two-d ... 03359.html
https://gmatclub.com/forum/is-the-posit ... 76553.html
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What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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08 Feb 2017, 16:43
1
Thank for the kind words, Mike! (This is one of mine.)

Bunuel, those weren't the inspiration, at least on a conscious level, but it's great to see the convergent evolution; they are certainly excellent and similar questions.

I'm very surprised this is currently listed as sub-600 level. I debated how difficult to go with this one; I could have done something like p - 8, p, p + 20 and gotten the same answer since p + 20 = p + 8 + 12 will be the same remainder w.r.t. 3 as p + 8.
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Re: What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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09 Feb 2017, 09:07
mikemcgarry wrote:
ziyuenlau wrote:
What is the value of prime number $$p$$?

(1) $$p+8$$ is prime.
(2) $$p−8$$ is prime.

Dear ziyuenlau,

I'm happy to respond.

This is a devilishly clever and difficult problem. Let's jump to the case in which we are considering both statements together.

Here are a few number sense rules.
1) In a set of three consecutive integers, one is always divisible by 3.
2) In a set of three evenly spaced integers, one is always divisible by 3.
(These rules generalize from 3 to n.)

The numbers (p - 8), p, and (p + 8) are three evenly spaced numbers, so it absolutely must be true that one of them is divisible by 3. The only way all three of them can be prime is if the one that is divisible by 3 is 3 itself! Thus:
p - 8 = 3
p = 11
p + 8 = 19
If p takes the value p = 11, this is the only way that all three numbers can be prime simultaneously. Thus, the value of p is unique determined. Combined the statements are sufficient. OA = (C).

Veritas wrote a truly beautiful problem here!

Does all this make sense?
Mike

Hi mikemcgarry,

I did not understand the rule well.

When you say-- the numbers (p - 8), p, and (p + 8) are three evenly spaced numbers, so it absolutely must be true that one of them is divisible by 3

even spaced means they are at the equal distance from preceding and succeeding term, or distance should be even as well?

For instance--
31 34 37

3 3

but none of them is divisible by 3; I think I misunderstood the concept. Please also explain how this rule is generalized from 3 to n. sorry for the silly question.
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What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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09 Feb 2017, 09:15
1
Good catch, AR15J! To clarify, the rule in question only applies to an evenly spaced set where the spacing isn't itself a multiple of 3.

More broadly, the general rule is this: Any list of N evenly spaced integers will contain a multiple of N as long as the spacing has no factors besides 1 in common with N. (In fact, it will contain exactly one such multiple.)

Try it!
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What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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09 Feb 2017, 11:14
AnthonyRitz wrote:
Good catch, AR15J! To clarify, the rule in question only applies to an evenly spaced set where the spacing isn't itself a multiple of 3.

More broadly, the general rule is this: Any list of N evenly spaced integers will contain a multiple of N as long as the spacing has no factors besides 1 in common with N. (In fact, it will contain exactly one such multiple.)

Try it!

Thanks AnthonyRitz for clearing the doubt

In GMAT also, evenly spaced means equal distance, below set is also evenly spaced?Please confirm

31,34,37
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Re: What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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09 Feb 2017, 13:05
1
AR15J wrote:
AnthonyRitz wrote:
Good catch, AR15J! To clarify, the rule in question only applies to an evenly spaced set where the spacing isn't itself a multiple of 3.

More broadly, the general rule is this: Any list of N evenly spaced integers will contain a multiple of N as long as the spacing has no factors besides 1 in common with N. (In fact, it will contain exactly one such multiple.)

Try it!

Thanks AnthonyRitz for clearing the doubt

In GMAT also, evenly spaced means equal distance, below set is also evenly spaced?Please confirm

31,34,37

AR15J,

Yes, that is correct. 31, 34, 37 is evenly spaced. (But it will never contain a multiple of 3 no matter how long you continue, since the spacing is 3 and since at least one element is not a multiple of 3.) (But if, for instance, you include at least one more element in either direction, you will be guaranteed a multiple of 4.)
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Re: What is the value of prime number p ? (1) p+8 is prime.  [#permalink]

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27 Feb 2018, 20:22
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Re: What is the value of prime number p ? (1) p+8 is prime. &nbs [#permalink] 27 Feb 2018, 20:22
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