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what is the value of x?

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what is the value of x?

by revoltangel » Tue Jan 06, 2009 8:42 am
If x is a prime number, what is the value of x?
(1) x < 15
(2) (x-2) is a multiple of 5

OA: E


Where did I go wrong? :
(1) x could be 2, 3, 5, etc... NOT SUFF
(2) (x-2) = 5k
x= 5k + 2
x could be 7, 17, 37, etc... NOT SUFF

Both together: X=7 because it's the only one less than 15, so C?
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by rajataga » Tue Jan 06, 2009 9:02 am
think on the negative side....

even -3 is a prime number and (x-2) = (-5) is a multiple of 5...

hence, E
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by vikz_316 » Tue Jan 06, 2009 9:08 am
A silly doubt, but is 0 a possible multiple of 5?
Then 2 also satisfies the condition.
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by cramya » Tue Jan 06, 2009 9:20 am
think on the negative side....

even -3 is a prime number and (x-2) = (-5) is a multiple of 5...

hence, E
Prime numbers are always positive. I dont think we classify -3,-5,-2 as primes

(2) (x-2) = 5k
x= 5k + 2
x could be 7, 17, 37, etc... NOT SUFF
Putting k=0 is what u may have missed.


A silly doubt, but is 0 a possible multiple of 5?
Then 2 also satisfies the condition
As Vikz very rightly said both 2 and 7 satisfy the 2 statements together
2-2 = 0 multiple of 5 (We almost always forget that 0 is a multiple of evry number in DS multiples concept problems).

7-2 = 5 is also a multiple of 5

Hence E)

Is this a problem from the Kaplan free full length exam? I remember seeing something like this.

Hope this helps!
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by revoltangel » Tue Jan 06, 2009 9:59 am
cramya wrote: Is this a problem from the Kaplan free full length exam? I remember seeing something like this.

Hope this helps!
Yes it is! Thanks for your help--I did not think of 0 as a multiple.
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by rajataga » Tue Jan 06, 2009 10:01 am
cramya wrote:
Prime numbers are always positive. I dont think we classify -3,-5,-2 as primes

yup, your right....

came across an old thread about this also, where Stacey and Ian have commented confirming this.
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