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rajatvmittal: Master | Next Rank: 500 Posts; Posts: 110; Joined: Sat Feb 11, 2012 5:01 am; Thanked: 2 times

by rajatvmittal » Sat Feb 16, 2013 10:07 pm

A Wagstaff prime is a prime number p such that p=((2q)+1)/3, when q is another prime. If p and q are positive integers, is p a Wagstaff prime? (q is a power to 2)

p=q
q=3

i chose D

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Source: Beat The GMAT — Data Sufficiency | Sat Feb 16, 2013 10:07 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Feb 17, 2013 7:50 am

rajatvmittal wrote:A Wagstaff prime is a prime number p such that p=((2^q)+1)/3, when q is another prime. If p and q are positive integers, is p a Wagstaff prime? (q is a power to 2)

1) p=q
2) q=3

i chose D

Note: Use "^" to indicate powers.

Target question: Is p a Wagstaff prime?

Given: p=((2^q)+1)/3
p and q must be a prime

Statement 1: p=q
Replace q with p to get: p=((2^p)+1)/3
Multiply both sides by 3 to get: 3p=(2^p)+1
Rearrange to get: 3p - 1 = 2^p
There are 2 possible solutions here: p=1 and p=3. Let's examine each:
p=1: No good, since we're told that p and q must be prime.
p=3: Good, since this means p and q are both prime.
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: q=3
If q=3, then p must equal 3 (which is prime)
Since we can answer the target question with certainty, statement 2 is SUFFICIENT.

Answer = D

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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rajatvmittal: Master | Next Rank: 500 Posts; Posts: 110; Joined: Sat Feb 11, 2012 5:01 am; Thanked: 2 times

by rajatvmittal » Sun Feb 17, 2013 8:14 am

i got d too.

but rxplanation says: -
Solution: B

Start with the easier statement first. If p = q, p and q could be any integer, but to answer the question we must know if both p and q are prime; statement (1) is INSUFFICIENT. Statement(2:q = 3. Now plug it into the formula given. ((2^3)+1)/3 = 9/3=3, so p = 3. This is prime, so statement (2) is SUFFICIENT on its own; (B).

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Feb 17, 2013 8:20 am

rajatvmittal wrote:i got d too.

but rxplanation says: -
Solution: B

Start with the easier statement first. If p = q, p and q could be any integer, but to answer the question we must know if both p and q are prime; statement (1) is INSUFFICIENT. Statement(2:q = 3. Now plug it into the formula given. ((2^3)+1)/3 = 9/3=3, so p = 3. This is prime, so statement (2) is SUFFICIENT on its own; (B).

I don't quite follow this solution.
If we say that statement 1 is insufficient, then there must be more than 1 possible value of p, such that one value is prime and the other value is not prime. What are these two possible values?

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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rajatvmittal: Master | Next Rank: 500 Posts; Posts: 110; Joined: Sat Feb 11, 2012 5:01 am; Thanked: 2 times

by rajatvmittal » Sun Feb 17, 2013 8:23 am

Hi Brent

I got the same question in my mind when i looked at the explanation. I too dint quite understand it and that is the reason I posted it on the forum.

So can i say that answer is D not B

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Feb 17, 2013 8:31 am

rajatvmittal wrote:Hi Brent

I got the same question in my mind when i looked at the explanation. I too dint quite understand it and that is the reason I posted it on the forum.

So can i say that answer is D not B

Yes, I think the answer is D.
Unless I'm overlooking something (wouldn't be the first time

).

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Sun Feb 17, 2013 9:03 am

This question makes no logical sense, or at least it doesn't if the OA is not C. It may be easiest to illustrate why it is nonsensical with a simpler example. If a question reads:

An odd integer is an integer k such that k = 2m + 1, where m is an integer. If k and m are integers, is k odd?
1. m = 5
2. k = 11

Notice here that the first part of the question, which tells us k is odd if k = 2m + 1 for some integer m, is just a definition of an odd number. In a definition, the letters are just placeholders; it doesn't matter if the definition uses 'k' and 'm' or if it uses 'x' and 'y'. The first part of the question could just as well read "An odd integer is an integer x such that x = 2y + 1 where y is an integer". It is most certainly not a fact about k and m; if you assume it is, then you're assuming in advance precisely what you are supposed to prove, and you'd be committing the 'begging the question' fallacy. So in the question above, the answer is B, since the value of m has nothing to do with anything.

So back to the question above, it starts by defining a Wagstaff prime as one which is equal to (2^q + 1)/3, where q is a prime. But this is just the definition of a Wagstaff prime; it most certainly is not a relationship we should assume holds between p and q, and if we do, we're assuming the answer to the question before we get started. Since letters in definitions are just placeholders, logically, the question is the identical to this one:

A Wagstaff prime is a prime number z such that z=((2^y)+1)/3, when y is another prime. If p and q are positive integers, is p a Wagstaff prime?
1. p=q
2. q=3

from which the answer is very clearly C.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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mohitsharda: Senior | Next Rank: 100 Posts; Posts: 64; Joined: Sat Aug 01, 2009 3:13 amFollowed by: 1 member; Thanked: 5 times; GMAT Score:740

by mohitsharda » Mon Feb 18, 2013 3:18 am

I think the answer should be C as well.
A way the answer could be B is if the question say q is another positive integer instead of "q is another prime."

Here's why.

from statement 1, we decipher that p = 1 or 3 only. Since p = q, q = 1 or 3 only. If the statement was q is a positive integer, then p may (if equal to 3) or may not be (if equal to 1) a wagstaff prime. That would make this statement insufficient

Statement 2 would remain sufficient.

But since the question says q is a prime, no point thinking too much, right?
:twisted:

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subhakam: Senior | Next Rank: 100 Posts; Posts: 38; Joined: Mon Jun 21, 2010 12:37 pm

by subhakam » Mon Feb 18, 2013 1:05 pm

Brent@GMATPrepNow wrote:
rajatvmittal wrote:A Wagstaff prime is a prime number p such that p=((2^q)+1)/3, when q is another prime. If p and q are positive integers, is p a Wagstaff prime? (q is a power to 2)

1) p=q
2) q=3

i chose D
Note: Use "^" to indicate powers.

Target question: Is p a Wagstaff prime?

Given: p=((2^q)+1)/3
p and q must be a prime

Statement 1: p=q
Replace q with p to get: p=((2^p)+1)/3
Multiply both sides by 3 to get: 3p=(2^p)+1
Rearrange to get: 3p - 1 = 2^p
There are 2 possible solutions here: p=1 and p=3. Let's examine each:
p=1: No good, since we're told that p and q must be prime.
p=3: Good, since this means p and q are both prime.
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: q=3
If q=3, then p must equal 3 (which is prime)
Since we can answer the target question with certainty, statement 2 is SUFFICIENT.

Answer = D

Cheers,
Brent

Hello Brent - I did not understand how you factorized 3p - 1 = 2^p in to 1 and 3 as the solutions for the equation ? I did get D because in both cases P would not be prime but was stuck in factorizing the equation (took an educated guess)

Many thanks
Subhakam

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