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If x is a prime number, what is x? (1) x < 15 (2) x-2 is

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If x is a prime number, what is x? (1) x < 15 (2) x-2 is [#permalink] New post 20 Aug 2008, 03:02
If x is a prime number, what is x?

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

I got C, but Kaplan says the answer is E because 0 is also a multiple of 5. Don't multiples of a number start from that number and not include 0???

Last edited by lionheart187 on 20 Aug 2008, 08:16, edited 1 time in total.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 03:10
lionheart187 wrote:
If x is a prime number, what is x?

(1) x < 15
(2) x is a multiple of 5

I got C, but Kaplan says the answer is E because 0 is also a multiple of 5. Don't multiples of a number start from that number and not include 0???


0 is indeed a multiple of 5. However, we are told that x is a prime number, information that excludes the possiblility that x is 0.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 03:17
a multiple of an integer is the product of that integer with another integer.

0 is a multiple of every integer.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 05:54
If x is a prime number then it has be greater than 1. Thus, answer should be C.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 05:58
Isn't the answer B ?

If x is a prime number (2 or higher) and x is a multiple of 5, then x = 5.

Of course, (1) is insufficient becuase x could be 2, 3, 5, 7, or 11. But 5 is the only multiple of 5 that is also a prime number, hence B.

lionheart187 wrote:
If x is a prime number, what is x?

(1) x < 15
(2) x is a multiple of 5

I got C, but Kaplan says the answer is E because 0 is also a multiple of 5. Don't multiples of a number start from that number and not include 0???

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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 06:09
jallenmorris wrote:
Isn't the answer B ?

If x is a prime number (2 or higher) and x is a multiple of 5, then x = 5.

Of course, (1) is insufficient becuase x could be 2, 3, 5, 7, or 11. But 5 is the only multiple of 5 that is also a prime number, hence B.

lionheart187 wrote:
If x is a prime number, what is x?

(1) x < 15
(2) x is a multiple of 5

I got C, but Kaplan says the answer is E because 0 is also a multiple of 5. Don't multiples of a number start from that number and not include 0???


We are on the same boat.. agree with you.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 06:20
Quote:
If x is a prime number, what is x?

(1) x < 15
(2) x is a multiple of 5


I like the approach Durgesh uses for these problems. When the question states "If...", make it where that condition of "if" creates the universe of numbers which we will deal with when we get to the statements.

So, if x is a prime number creates a universe of only prime numbers. And "What is x?" means we're trying to determine if we can narrow the available choice of x down to a single value.

Prime numbers...2, 3, 5, 7, 11, 13, 17, 19, 23. We've gone high enough since we can see from (1) that one condition imposed is x < 15.

so (1), if x < 15, we still have five options, 2, 3, 5, 7, and 11. This doens't narrow the choices down to a single value. INSUFFICIENT.

(2) X is a multiple of 5. We're still dealing with the "universe" of prime numbers. The only multiple of 5 we see in the "universe" is 5. so the answer must be B.

Since one of the statements is sufficient alone, we do not need to consider the statements together.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 06:22
I agree with jallen & suresh.

B

0 is not a prime.
A prime number is a positive integer that has exactly two factors, 1 and the number itself.

We know 0 is neither a positive nor a negative number. 0 is a neutral number. So, it is not a prime number.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 07:20
phew, for a second there I thought I was the only one who thought that B should be the answer ...

If x is a multiple of 5, well, the only PRIME multiple of 5 is 5 .... 0 isnt a prime #
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 07:39
jallenmorris wrote:
Isn't the answer B ?

If x is a prime number (2 or higher) and x is a multiple of 5, then x = 5.

Of course, (1) is insufficient becuase x could be 2, 3, 5, 7, or 11. But 5 is the only multiple of 5 that is also a prime number, hence B.

lionheart187 wrote:
If x is a prime number, what is x?

(1) x < 15
(2) x is a multiple of 5

I got C, but Kaplan says the answer is E because 0 is also a multiple of 5. Don't multiples of a number start from that number and not include 0???

So sorry, (2) should be x-2 is a multiple of 5.

Thanks for noticing!
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 07:41
Isn't the answer still B?

if x-2 is a multiple of 5, and x is prime, then x = 7.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 07:43
lionheart187 wrote:
jallenmorris wrote:
Isn't the answer B ?

If x is a prime number (2 or higher) and x is a multiple of 5, then x = 5.

Of course, (1) is insufficient becuase x could be 2, 3, 5, 7, or 11. But 5 is the only multiple of 5 that is also a prime number, hence B.

lionheart187 wrote:
If x is a prime number, what is x?

(1) x < 15
(2) x is a multiple of 5

I got C, but Kaplan says the answer is E because 0 is also a multiple of 5. Don't multiples of a number start from that number and not include 0???

So sorry, (2) should be x-2 is a multiple of 5.

Thanks for noticing!


please edit the original question too.

Then I will go with E.

you will get two options when combined
x=2 x-2 = 0 multiple of 5
x=7 x-2=5 multiple of 5
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Last edited by x2suresh on 20 Aug 2008, 07:44, edited 1 time in total.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 07:44
Nice catch suresh.
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Re: Error on Kaplan CAT? [#permalink] New post 20 Aug 2008, 12:05
IF x-2 is a multiple of 5 , shouldn't x = 7 rather than 5 and if that is the case wouldn't x have the following values
x = 7,17 etc.

How can 2) be sufficient
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Re: Error on Kaplan CAT? [#permalink] New post 31 Aug 2008, 01:31
Page 14 of the NUMBER PROPERTIES GUIDE by MGMAT states the following

"Multiples multiply out from an integer and are therefore greater than or equal to that integer."

Therefore 0 would not be a multiple of 5! Correct me if Im wrong!
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Re: Error on Kaplan CAT? [#permalink] New post 31 Aug 2008, 01:39
Definition of Multiples

The products of a number with the natural numbers 1, 2, 3, 4, 5, ... are called the multiples of the number.
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Re: Error on Kaplan CAT? [#permalink] New post 31 Aug 2008, 07:13
lionheart187 wrote:
Page 14 of the NUMBER PROPERTIES GUIDE by MGMAT states the following

"Multiples multiply out from an integer and are therefore greater than or equal to that integer."

Therefore 0 would not be a multiple of 5! Correct me if Im wrong!


That is not true- if that's what the MGMAT guide says, they have it wrong, as a glance at any proper math book will demonstrate. The multiples of 5 are all numbers 5*x, where x is an integer, positive or negative (or zero):

...-10, -5, 0, 5, 10, ...

On the real GMAT, however, questions about divisibility and multiples are almost always restricted to positive integers- they will begin questions by saying 'If x is a positive integer...', so you probably won't need to worry about negative multiples on the test.
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Re: Error on Kaplan CAT?   [#permalink] 31 Aug 2008, 07:13
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