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If X is a prime number, what's the value of X? (1) X < 15

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If X is a prime number, what's the value of X? (1) X < 15 [#permalink]

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New post 12 Jun 2009, 17:52
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If X is a prime number, what's the value of X?

(1) X < 15
(2) (X-2) is a multiple of 5.

The correct answer is E....why?

I think it's C and here's how I solved it

from (1), X can be 2,3,5,7,11 and 13
from (2) X is 7 because 7 - 2 = 5 which is a multiple of 5

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Re: prime numbers - DS [#permalink]

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New post 12 Jun 2009, 21:19
The answer can be E here:

1 is not sufficient.
2 is not sufficient either.

Together take values of X as 2, then 2-2 = 0 , which is a multiple of 5 (as 0*5=0). (0 is a multiple of 5).
X=7 will also give a multiple of 5.

Thus E.

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Re: prime numbers - DS [#permalink]

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New post 12 Jun 2009, 21:22
Hi,

It is because there can be many values of x that fulfills the second statement.

For example:
17-2 = 15, multiple of 5 too.

We do not know whether X is 15 or 5, hence insufficient.

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Re: prime numbers - DS [#permalink]

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New post 12 Jun 2009, 21:28
MontrealGirl wrote:
If X is a prime number, what's the value of X?

(1) X < 15
(2) (X-2) is a multiple of 5.

The correct answer is E....why?

I think it's C and here's how I solved it

from (1), X can be 2,3,5,7,11 and 13
from (2) X can be 17, 37, and similar.

Combining both X is 7 because 7 -
2 = 5 which is a multiple of 5


You are right I got to the same conclusion. What is the source of this question and OE. Or may be they are not considering 5 as multiple of 5.

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Re: prime numbers - DS [#permalink]

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New post 12 Jun 2009, 21:30
X cannot be 17 because X<15. Hence the values possible for X are 2 and 7 (prime nos) where X-2 is a multiple of 5.

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Re: prime numbers - DS [#permalink]

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New post 12 Jun 2009, 21:31
gmatprep09 wrote:
The answer can be E here:

1 is not sufficient.
2 is not sufficient either.

Together take values of X as 2, then 2-2 = 0 , which is a multiple of 5 (as 0*5=0). (0 is a multiple of 5).
X=7 will also give a multiple of 5.

Thus E.


Not sure but seems the only plausible explanation. Thanks

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Re: prime numbers - DS [#permalink]

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New post 13 Jun 2009, 06:54
Jozu wrote:
Hi,

It is because there can be many values of x that fulfills the second statement.

For example:
17-2 = 15, multiple of 5 too.

We do not know whether X is 15 or 5, hence insufficient.


but from (1), we know that the prime number should be less than 15

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Re: prime numbers - DS [#permalink]

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New post 13 Jun 2009, 06:54
humans wrote:
MontrealGirl wrote:
If X is a prime number, what's the value of X?

(1) X < 15
(2) (X-2) is a multiple of 5.

The correct answer is E....why?

I think it's C and here's how I solved it

from (1), X can be 2,3,5,7,11 and 13
from (2) X can be 17, 37, and similar.

Combining both X is 7 because 7 -
2 = 5 which is a multiple of 5


You are right I got to the same conclusion. What is the source of this question and OE. Or may be they are not considering 5 as multiple of 5.


Kaplan practice test

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Re: prime numbers - DS [#permalink]

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New post 16 Jun 2009, 14:16
[quote="MontrealGirl"]If X is a prime number, what's the value of X?

(1) X < 15
(2) (X-2) is a multiple of 5.

obviously each alone is not suff

both

x-2 = 5a

x = 5a-2.. x = 3 or13,23.....etc

from one
x<15 ie x = 2or3,5,7,11,13

the intersection of the 2 one and two is 3,13 we cant determine one single value...thus E

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Re: prime numbers - DS [#permalink]

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New post 17 Jun 2009, 18:36
yezz wrote:
MontrealGirl wrote:
If X is a prime number, what's the value of X?

(1) X < 15
(2) (X-2) is a multiple of 5.

obviously each alone is not suff

both

x-2 = 5a

x = 5a-2.. x = 3 or13,23.....etc

from one
x<15 ie x = 2or3,5,7,11,13

the intersection of the 2 one and two is 3,13
we cant determine one single value...thus E


A small correction note for a typo: x = 5a + 2.. x = 2 or 7 or 12 or 17 or so on....
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Re: prime numbers - DS [#permalink]

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New post 18 Jun 2009, 08:59
gmatprep09 wrote:
The answer can be E here:

1 is not sufficient.
2 is not sufficient either.

Together take values of X as 2, then 2-2 = 0 , which is a multiple of 5 (as 0*5=0). (0 is a multiple of 5).
X=7 will also give a multiple of 5.

Thus E.


Leaving out zero is the reason why we will get C.

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Re: prime numbers - DS   [#permalink] 18 Jun 2009, 08:59
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If X is a prime number, what's the value of X? (1) X < 15

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