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If 9 is a factor of 2x, then which of the following may not

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If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 25 Dec 2004, 13:56
3
9
00:00
A
B
C
D
E

Difficulty:

  75% (hard)

Question Stats:

52% (01:56) correct 48% (01:35) wrong based on 434 sessions

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If 9 is a factor of 2x, then which of the following may not be an integer?

A. 6x/54 + 2x/3

B. (4x-18)/9

C. (2x+27)/9

D. (81-4x^2)/81

E. (2x-3)/3

The answer and my question are posted below.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 25 Dec 2004, 14:06
I got the right answer A after plugging in 18 for x.

While going through Kaplan's explanation, I couldn't understand the part about choice A.

"6x/54 + 2x/3 = 3/6 * 2x/9 + 2x/3.

Since 9 is a factor of 2x, 3 is also factor of 2x. So 2x/3 is an integer, as is 2x/9. But 3/6 = 1/2, so if 2x/9 is not even, the expression 3/6 * 2x/9 will not be even."

But, isn't 2x/9 always even? 9 is a factor of 2x, which means that we could have 2(9), 2(2)(9), 2(3)(9), and so forth. The 9 in the denominator cancels out the 9 in the numerator. So we are left with 2 * something. So 2x/9 is even.

Your thoughts?
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 25 Dec 2004, 17:25
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The question tells us that 9 is a factor of 2x.

Therefore, 2x = 9k where k is any integer.

2x = 9k. Therefore 6x = 27k.

But 6 is an even number. Therefore k also has to be an even number. But if k necessarily has to be an even number, then 6x must be divisible by 54.

Therefore, we can say that 6x = 54m for some integer m....... (1)

Since 2x is divisible by 9, it is also divisible by 3........... (2)

By (1) and (2), we can see that 6x/54 + 2x/3 is an integer.

So, the answer is NOT choice A.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 25 Dec 2004, 19:57
A.

Stem: 2x/9 = n where n is any integer.

a. 6x/54 + 2x/3 (lets look at other choices)

b. (4x-18)/9 = 4x/9 -18/9 = 2.2x/9 - 2 = 2n -2 = integer

c. (2x+27)/9 = 2x/9 + 27/9 = n+3 = integer

d. (81-4x^2)/81 = 81/81 - (2x/9)^2 = 1-n^2 = integer

e. (2x-3)/3 = 2x/3 - 3/3 = 2x/3 - 1 = integer (since 2x is divisible by 9, it is also divisible by 3)

The only choice that's left is A.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 26 Dec 2004, 15:21
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Questor,

If you plug in 18 for x, then -

(6 * 18)/54 + (2 * 18)/3 = 2 + 12 = 14 (which is an integer).

As my earlier post showed, choice A cannot be the correct answer.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 27 Dec 2004, 11:18
my choice is D. the tricky part of the question is that D is written as (81-4x^2)/81 . With the (x-y), we must solve the (81-4x^2) before doing the division with 81. The result is often a fraction.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 28 Dec 2004, 17:11
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NM, the question says "which of the following may not be an integer?"

May Not is the key word in the question.
Lets look at choice A:
6x/54 + 2x/3 = (3/6)(2x/9) + 2x/3 = (1/2)(2x/9) + 2x/3

So, as long as 2x/9 is a multiple of two it will be an integer. In your case you are using 2(18)/9 = 4, a multiple of two and hence the answer is an integer.

It does not say x is an integer, so if you pick x=13.5 (2x=27 is divisible by 9), we get
6x/54 + 2x/3 = 1.5 + 9 = 10.5, not an integer.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 28 Dec 2004, 18:22
All

the answer to this is simple since 9 is a factor of 2x that means x can be and integer then its a multiple of 9 itself then choices A - E will hold good and we will always get an integer.

BUT the problem does not say that x is an intger so take x = 4.5 so 2x will be 9 and 9 is a factor of 9 plug in the choices A - E you will only get D as an decimal value not an integer

ANSWER IS D

rxs0005
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 28 Dec 2004, 22:56
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Hi. Thanks for the explanations, guys! I'll check the CD again to make sure that I didn't make any typos in my original.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 31 Dec 2004, 18:17
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rxs0005 wrote:
All

the answer to this is simple since 9 is a factor of 2x that means x can be and integer then its a multiple of 9 itself then choices A - E will hold good and we will always get an integer.

BUT the problem does not say that x is an intger so take x = 4.5 so 2x will be 9 and 9 is a factor of 9 plug in the choices A - E you will only get D as an decimal value not an integer

ANSWER IS D

rxs0005


rxs0005, I dont see how you get D.

Lets use x=4.5 as you have suggested:
a. 6x/54 + 2x/3 = 6(4.5)/54 +9/3 = 3.5 (NOT an integer)

b. (4x-18)/9 = (4(4.5)-18)/9 = (18-18)/9 = 0 (an integer)

c. (2x+27)/9 = (9+27)/9 = 4 (an integer)

d. (81-4x^2)/81 = (81 - 4(4.5)^2)/81 = (81-81)/81 = 0 (an integer)

e. (2x-3)/3 = (9-3)/3 = 2 (an integer)
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 01 Jan 2005, 11:58
RX you are wrong. the answer is A, you have to think of in terms of prime facrtos

2X/9 is like 2X/3.3 right?

so lets look at answer choice A

6x/54 is not an integer!

6X = 2.3.X right?
54= 3.3.2.3

6x/54 = 2.3.x/3.3.2.3 as you can see, 2.3 in the numerator cancels out with the ones in the denominator, right? you are left with X/3.3 which is never stated to be an integer, remember 2x/3.3 is an integer not x/3.3!



rxs0005 wrote:
All

the answer to this is simple since 9 is a factor of 2x that means x can be and integer then its a multiple of 9 itself then choices A - E will hold good and we will always get an integer.

BUT the problem does not say that x is an intger so take x = 4.5 so 2x will be 9 and 9 is a factor of 9 plug in the choices A - E you will only get D as an decimal value not an integer

ANSWER IS D

rxs0005
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 01 Jan 2005, 15:10
A it is.

What you learn is that 2x = 9N where N is equal to or greater than 1.

if 2x = 9, then A is not an integer, for BCDE 2x=9 yields to integer results...
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 11 Aug 2015, 13:21
If 9 is a factor of 2x => 2x = 9k, where k = 0, 1, etc

6x/54 + 2x/3 can be rewritten in terms of k as:
27k/54 + 9k/3

which is k/2 + 3k

the 3k will always be an integer, but the first term only works for even k.

alternatively, if you just plugged in k=1 => 2x=9 => x=4.5 you would have arrived at A aswell
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If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 29 Dec 2015, 05:43
The answer is A.

The questions provides that 2x is divisible by 9, so x is either a multiple of 9 or x = 9/2 = 4.5

If x = 4.5 then 6x/54 = (6*4.5)/ (6*9) = 1/2. Not an integer.

In all other cases when the equations are simplified, 2x is always the numerator and 9 or 3 always the denominator making the answer always an integer.
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 09 Oct 2016, 10:14
gayathri wrote:
NM, the question says "which of the following may not be an integer?"

May Not is the key word in the question.
Lets look at choice A:
6x/54 + 2x/3 = (3/6)(2x/9) + 2x/3 = (1/2)(2x/9) + 2x/3

So, as long as 2x/9 is a multiple of two it will be an integer. In your case you are using 2(18)/9 = 4, a multiple of two and hence the answer is an integer.

It does not say x is an integer, so if you pick x=13.5 (2x=27 is divisible by 9), we get
6x/54 + 2x/3 = 1.5 + 9 = 10.5, not an integer.


Thank you very much, I wrongly assumed (and i should be beat for it because I knew better!) that x is an integer...I should've picked a more uncommon value for x in which 2x/9 still holds true. Again, thank you
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Re: If 9 is a factor of 2x, then which of the following may not [#permalink]

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New post 11 Feb 2018, 13:14
Hi All,

Tougher GMAT questions are sometimes designed to test the thoroughness of your thinking. This prompt NEVER stated that X was an integer; it even goes so far as to state that 9 is a factor of 2X (not that 9 is a factor of X). When a prompt is built around some quirky language or 'design elements', it's often worthwhile to ask yourself what you really KNOW and what you don't know. As has been pointed out in several of the explanations, thinking about X=4.5 is the easiest way to get to the correct answer (even though 4.5 isn't the easiest value that fits the given description).

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Re: If 9 is a factor of 2x, then which of the following may not   [#permalink] 11 Feb 2018, 13:14
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