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Re: dS prime numbers [#permalink]
28 Mar 2010, 06:01

aljatar wrote:

Please need some help with this one ... thanks a lot

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 3,912

1: p has to be 100th prime number starting from 2 (1st prime number) sufficient. 2: all the prime numbers between 1 and 3912 can be found and the number of prime number will be what we need. Sufficient. Hence D.

PS: This is a DS question so we do not need to calculate the p till end. calculation will take some time but no need to do.

Re: dS prime numbers [#permalink]
16 Apr 2010, 10:25

bangalorian2000 wrote:

aljatar wrote:

Please need some help with this one ... thanks a lot

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 3,912

1: p has to be 100th prime number starting from 2 (1st prime number) sufficient. 2: all the prime numbers between 1 and 3912 can be found and the number of prime number will be what we need. Sufficient. Hence D.

PS: This is a DS question so we do not need to calculate the p till end. calculation will take some time but no need to do.

Using Stmt 1, it's pretty straight forward.

Using Stmt 2, let n = no. of prime numbers between 1 and 3912. How can we be sure that n is a prime number and it is equal to p? Do we have to assume so in questions like these?

Re: dS prime numbers [#permalink]
16 Apr 2010, 22:57

D is correct. Both statements are sufficient by itself.

We can find the prime no. using the given information in each statement. But we need not find that number, just need to know the sufficiency ! _________________

"Success is going from failure to failure without a loss of enthusiam." - Winston Churchill

Re: dS prime numbers [#permalink]
17 Apr 2010, 04:25

Expert's post

PhunsukWangdu wrote:

bangalorian2000 wrote:

aljatar wrote:

Please need some help with this one ... thanks a lot

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 3,912

1: p has to be 100th prime number starting from 2 (1st prime number) sufficient. 2: all the prime numbers between 1 and 3912 can be found and the number of prime number will be what we need. Sufficient. Hence D.

PS: This is a DS question so we do not need to calculate the p till end. calculation will take some time but no need to do.

Using Stmt 1, it's pretty straight forward.

Using Stmt 2, let n = no. of prime numbers between 1 and 3912. How can we be sure that n is a prime number and it is equal to p? Do we have to assume so in questions like these?

Stem and the statements are ALWAYS providing us with correct information.

If it turns out that the quantity of primes between 1 and 3912 is not the prime number itself, this will mean that the question is flawed. GMAT wouldn't give us such question then.

Stem says p is a prime number. Statement (2) says that "there are a total of p prime numbers between 1 and 3912". So yes the # of primes between 1 and 3912 MUST be prime number itself. We don't know what number it is, but we can calculate it, hence we can calculate p, hence (2) is also sufficient.

Re: dS prime numbers [#permalink]
22 Nov 2010, 15:09

statement A is not sufficient...consider this: how many prime numbers are between 1 and 14 and between 1 and 16,,,same number..you can not conclude anything from statement like there 100 prime less than p+1... B is sufficient..

Re: dS prime numbers [#permalink]
23 Nov 2010, 04:33

1

This post received KUDOS

Expert's post

lhich2000 wrote:

statement A is not sufficient...consider this: how many prime numbers are between 1 and 14 and between 1 and 16,,,same number..you can not conclude anything from statement like there 100 prime less than p+1... B is sufficient..

OA for this question is D, not B.

Statement (1) is says that there are 100 primes in the range between 1 and P+1, so this statement basically says that P is the 100th prime --> we can determine the single numerical value of P (we can find 100th prime). As this is DS question no matter what the actual value of P is, the fact that we can find it, is already sufficient. _________________

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