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Primes: If p is a prime number greater than 2, what is the v

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II Master | Next Rank: 500 Posts
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Primes: If p is a prime number greater than 2, what is the v

Post Sun Mar 23, 2008 6:15 pm
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    If p is a prime number greater than 2, what is the value of p ?

    (1) There are a total of 100 prime numbers between 1 and p+1

    (2) There are a total of p prime numbers between 1 and 3912

    Thanks.



    Last edited by II on Mon May 05, 2008 1:50 am; edited 1 time in total

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    Stuart Kovinsky GMAT Instructor
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    Post Mon Mar 24, 2008 11:15 am
    II wrote:
    If p is a prime number greater than 2, what is the value of p ?

    (1) There are a total of 100 prime numbers between 1 and p+1

    (2) There are a total of p prime numbers between 1 and 3912

    Thanks.
    (1) We could write out a list of the first 100 primes. Since there are 100 primes between 1 and p+1, p is the 100th prime on our list: sufficient.

    (2) We could write out all the primes between 1 and 3912 and count them: sufficient.

    Each of (1) and (2) is sufficient on its own: choose (D).

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    II Master | Next Rank: 500 Posts
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    Post Mon Mar 24, 2008 2:16 pm
    Easy as that ! Smile
    Thanks Stuart ...

    gmatguy16 Master | Next Rank: 500 Posts Default Avatar
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    Post Mon Mar 24, 2008 4:59 pm
    stuart isnt the answer b,
    in the sense says there are 3 prime numbers between 2 and 11 (p+1) ,but there are also 3 prime numbers between 2 and 8 ,2 and 9 ,2 and 10 , 2 and 11(since 8,9 and 10 are not prime)..so p could be 7 ,8,9 or 10 since question specifically asks for value of p.

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    Post Tue Mar 25, 2008 11:43 am
    gmatguy16 wrote:
    stuart isnt the answer b,
    in the sense says there are 3 prime numbers between 2 and 11 (p+1) ,but there are also 3 prime numbers between 2 and 8 ,2 and 9 ,2 and 10 , 2 and 11(since 8,9 and 10 are not prime)..so p could be 7 ,8,9 or 10 since question specifically asks for value of p.
    We know that p is a prime number, so we also know that p+1 isn't a prime number (the only consecutive primes are 2 and 3, and p certainly isn't 2 based on other info we have).

    So, in your example, the only number that would be a possible match would be p=7 and p+1=8.

    If we hadn't known that p was prime, then you'd be right, there would potentially be multiple values (we'd have to actually count them out to tell).

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    microke Senior | Next Rank: 100 Posts Default Avatar
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    Post Thu Jul 17, 2008 4:19 am
    Stuart, is it not necessary to have "unique" value of p from both the statements?

    One can write first 100 primes and can find P from statement 1. Meanwhile one can write all the primes between 1 and 3912. My question is what is the surety that the answers from both the statements give a unique p? How one can cross check for such big problems or is not necessary that two statements should point to the unique single number? I am somewhat confused....

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    Post Fri Jul 18, 2008 9:54 am
    microke wrote:
    Stuart, is it not necessary to have "unique" value of p from both the statements?

    One can write first 100 primes and can find P from statement 1. Meanwhile one can write all the primes between 1 and 3912. My question is what is the surety that the answers from both the statements give a unique p? How one can cross check for such big problems or is not necessary that two statements should point to the unique single number? I am somewhat confused....
    If a statement is sufficient by itself, you NEVER have to worry about the statements together.

    By design, any time that each statement gives you enough information, the two statements always agree on the answer. Otherwise, when you combined the statements (it always has to be possible to combine them, even though it's not always necessary), you'd get a null answer to the question - and there's always at least one answer.

    Here are a couple of examples of what you would NEVER see on the GMAT:

    What's the value of x?

    1) x + 14 = 23

    2) x - 14 = 23

    Each statement gives you a unique value for x, but they give conflicting values. Therefore, the above is an impossible (i.e. it would NEVER appear) GMAT question.

    Is x > 0?

    1) x > 20

    2) x < -3

    Looked at separately, each statement gives you a definite answer: from (1) we get a definite "yes" and from (2) we get a definite "no". However, on the GMAT that will NEVER happen, since taken together there are no possible values for x (i.e. there's no number that's BOTH greater than 20 AND less than -3). Hence, this would be another impossible (i.e. it would NEVER appear) GMAT question.

    On a side note, you can sometimes use the above rule to double check your work. If you decided that each statement was sufficient but you notice that each one gave you a different answer, then you can be certain that you made a mistake along the way.

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    microke Senior | Next Rank: 100 Posts Default Avatar
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    Post Fri Jul 18, 2008 8:44 pm
    Thanks Stuart, for clarifying the important concept in DS. Now I understood GMAC philosophy on DS. Very valuble input.....

    ohwell Junior | Next Rank: 30 Posts Default Avatar
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    Post Mon Apr 06, 2009 3:16 pm
    The part I stumbled on in this question is that nowhere does it say that the prime numbers need to be unique. I guess it can be somewhat inferred, but I was thinking what if all the prime numbers are all the same, who knows what p can ever be?

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    Post Mon Apr 06, 2009 6:59 pm
    ohwell wrote:
    The part I stumbled on in this question is that nowhere does it say that the prime numbers need to be unique. I guess it can be somewhat inferred, but I was thinking what if all the prime numbers are all the same, who knows what p can ever be?
    Whenever a question talks about "the numbers between x and y", it's referring to the number line, on which each number appears exactly once.

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    LulaBrazilia Master | Next Rank: 500 Posts Default Avatar
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    Post Tue Apr 07, 2009 8:51 pm
    Stuart Kovinsky wrote:
    II wrote:
    If p is a prime number greater than 2, what is the value of p ?

    (1) There are a total of 100 prime numbers between 1 and p+1

    (2) There are a total of p prime numbers between 1 and 3912

    Thanks.
    (1) We could write out a list of the first 100 primes. Since there are 100 primes between 1 and p+1, p is the 100th prime on our list: sufficient.

    (2) We could write out all the primes between 1 and 3912 and count them: sufficient.

    Each of (1) and (2) is sufficient on its own: choose (D).
    Good explanation

    LulaBrazilia Master | Next Rank: 500 Posts Default Avatar
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    Post Tue Apr 07, 2009 9:02 pm
    I just hope I'm never asked to find the sum of the first 100 prime numbers!

    austin Master | Next Rank: 500 Posts Default Avatar
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    Post Tue Apr 07, 2009 10:58 pm
    As Stuart rightly put it, we need not find the exact answer in DS (unlike PS). All we need to check is whether the statement(s) give us a 100% unique (consistent) answer.

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