Multiples of 5?

[anishprabhu]

if x is a prime number, what is the value of x?

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

OA E

[cramya]

I am getting C

Stmt I

x<15

many primes possible

INSUFF

Stmt II

x-2 = 15 * k

x= 15k+2

prime possible

2, 17

INSUFF

Together:

X IS A PRIME
X<15
X-2 MULTIPLE OF 15

Can only be 2

Must be missing something if the OA is indeed E

SUFF

[cramya]

If statement 2 is

x-2 is a multiple of 5 then it would be E

SINCE 2 AND 7 ARE POSSIBLE.

Either stmt II has a typo or the OA is incorrect. Just my thought.


Regards,
CR

[rolrol19]

yeah doesnt make much sense if it's 15

[karmayogi]

A.x < 15. Possible values of x = 2, 3, 5 ..13
B.x-2 is a multiple of 15
Therefore,
x-2 = n15
where n could be -1, 0, 1, 2, 3, …but, as x is a prime number, n should be non-negative i.e. 0, 1, 2, …

x = n15 + 2
For example, x could be 2, 17, 107…etc.

Combining A and B. x =2 is the only option possible answer.

IMO C

What’s the source?

[karmayogi]

However, if we replace 15 by 5 in above question. Then,
A: x = 2, 3, 5, 7...13
B: x = 2, 7, 17, 37...
A&B: x = 2 or 7

Hence, IMO E

[hijazim]

When I first check the question, I chose E right away. Because

1/ alone => many prime values for x / (13,11,7,5,2,...)
2/ alone => (x-2) is a multiple of 15 => x-2 > or = 15
=> x > or = 17

Is this right or we can say that 0 is a multiple of 15 so x=2.. But I guess we cant count 0 as multiple.. can we??

Together 1/ and 2/ => x < 15 and x > or = 17 ... Can't be

BUT, my concern is that whether GMAT put two contradicting statements in DS questions. Or may be i am wrong in the steps i did..

Please any suggestions..

[karmayogi]

When I first check the question, I chose E right away. Because

1/ alone => many prime values for x / (13,11,7,5,2,...)
2/ alone => (x-2) is a multiple of 15 => x-2 > or = 15
=> x > or = 17

Is this right or we can say that 0 is a multiple of 15 so x=2.. But I guess we cant count 0 as multiple.. can we??

Together 1/ and 2/ => x < 15 and x > or = 17 ... Can't be

BUT, my concern is that whether GMAT put two contradicting statements in DS questions. Or may be i am wrong in the steps i did..

Please any suggestions..

Check this https://en.wikipedia.org/wiki/Multiple_(mathematics)

[hijazim]

ok thanks karmayogi. So zero is a multiple of any number.

[Goal to 800]

if x is a prime number, what is the value of x?

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

OA E

Should it say "factor" rather than "multiple"

Fewer factors, More Multiples...

Multiples of 15 = 15, 30, 45, etc...

I understand why the answer would be (E) if it said "Factors" but I don't understand why it says multiples.

Thanks

[Goal to 800]

When I first check the question, I chose E right away. Because

1/ alone => many prime values for x / (13,11,7,5,2,...)
2/ alone => (x-2) is a multiple of 15 => x-2 > or = 15
=> x > or = 17

Is this right or we can say that 0 is a multiple of 15 so x=2.. But I guess we cant count 0 as multiple.. can we??

Together 1/ and 2/ => x < 15 and x > or = 17 ... Can't be

BUT, my concern is that whether GMAT put two contradicting statements in DS questions. Or may be i am wrong in the steps i did..

Please any suggestions..

Check this https://en.wikipedia.org/wiki/Multiple_(mathematics)

Thank you for the link...

It answers my question about multiples being negative..

"14, 49, 0 and -21 are multiples of 7 whereas 3 and -6 are not. This is because there are integers that 7 may be multiplied by to reach the values of 14, 49, 0, and -21, while there are no such integers for 3 and -6. Each of the products listed below, and in particular, the products for 3 and -6, is the only way that the relevant number can be written as a product of 7 and another real number:"

Nice.