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if x is prime?

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Source: — Data Sufficiency |
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by cramya » Sun Nov 09, 2008 9:59 am
I would go with B)

I understand the OA's after some discussion part :-) but going forward if you could post OA's using SPOILER function then that would be much appreciated

To find if x is prime

Stmt I

x is positive
x+2 is prime

Lets say x+2 = 2
x=4 (not prime)

x+3 = 5
x=3 (prime)

INSUFF

Stmt II

x-7 = prime number

Lets say x-7 = 2(ONLY EVEN PRIME)

x=9 not prime

2 IS THE ONLY EVEN PRIME. The rest of the primes are odd

So x-7 = prime becomes x-7 = odd prime
x = 7+odd prime

x will always be even and will not be a prime (since odd+odd = even)

B)



Hope I have not missed anything here!
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by stop@800 » Sun Nov 09, 2008 11:02 am
one more B
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by vittalgmat » Mon Nov 10, 2008 2:20 pm
cramya wrote:I would go with B)

I understand the OA's after some discussion part :-) but going forward if you could post OA's using SPOILER function then that would be much appreciated

To find if x is prime

Stmt I

x is positive
x+2 is prime

Lets say x+2 = 2
x=4 (not prime)

x+3 = 5
x=3 (prime)

INSUFF

Stmt II

x-7 = prime number

Lets say x-7 = 2(ONLY EVEN PRIME)

x=9 not prime

2 IS THE ONLY EVEN PRIME. The rest of the primes are odd

So x-7 = prime becomes x-7 = odd prime
x = 7+odd prime

x will always be even and will not be a prime (since odd+odd = even)

B)



Hope I have not missed anything here!

Hi cramya,
I dint understand how it is B.
from 2. we have x -7 = prime# p.
x -7 = p
x = p +7.
SO x is any number that is 7 away from a prime #.
eg x = 3 + 7. x = 10, Not a prime
x = 2 +7. x = 9.
X could be any +ve # including a prime. So insufficient.

So once cannot say if x is prime. 2 is insufficient.

Have i missed something ??

thanks
-Vittal
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by vittalgmat » Mon Nov 10, 2008 2:22 pm
vittalgmat wrote:
cramya wrote:I would go with B)

I understand the OA's after some discussion part :-) but going forward if you could post OA's using SPOILER function then that would be much appreciated

To find if x is prime

Stmt I

x is positive
x+2 is prime

Lets say x+2 = 2
x=4 (not prime)

x+3 = 5
x=3 (prime)

INSUFF

Stmt II

x-7 = prime number

Lets say x-7 = 2(ONLY EVEN PRIME)

x=9 not prime

2 IS THE ONLY EVEN PRIME. The rest of the primes are odd

So x-7 = prime becomes x-7 = odd prime
x = 7+odd prime

x will always be even and will not be a prime (since odd+odd = even)

B)



Hope I have not missed anything here!

Hi cramya,
I dint understand how it is B.
from 2. we have x -7 = prime# p.
x -7 = p
x = p +7.
SO x is any number that is 7 away from a prime #.
eg x = 3 + 7. x = 10, Not a prime
x = 2 +7. x = 9.
X could be any +ve # including a prime. So insufficient.

So once cannot say if x is prime. 2 is insufficient.

Have i missed something ??

thanks
-Vittal

OOPS!!! Got it!!. thanks Cramya.

-V
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