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# If y is an integer, is y^3 divisible by 9 ? (1) y is

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Manager
Joined: 09 Jul 2008
Posts: 107
Location: Dallas, TX
Schools: McCombs 2011
If y is an integer, is y^3 divisible by 9 ? (1) y is  [#permalink]

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02 Nov 2008, 11:12
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If y is an integer, is $$y^3$$ divisible by 9 ?

(1) y is divisible by 4
(2) y is divisible by 6

How can I rephrase the question stem ?

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VP
Joined: 30 Jun 2008
Posts: 1004

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02 Nov 2008, 12:02
kman wrote:
If y is an integer, is $$y^3$$ divisible by 9 ?

(1) y is divisible by 4
(2) y is divisible by 6

How can I rephrase the question stem ?

for $$y^3$$ to be divisible by 9, it has to be a multiple of 9.

(1) clearly not sufficient.
(2) y is divisible by 6, then y has at least one 3 as a factor. So $$y^3$$ will have at least 3³ which is divisible by 9

B ??
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Manager
Joined: 23 Aug 2008
Posts: 62

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02 Nov 2008, 12:15
kman wrote:
If y is an integer, is $$y^3$$ divisible by 9 ?

(1) y is divisible by 4
(2) y is divisible by 6

How can I rephrase the question stem ?

I'm going for B, and I would rephrase the question as below

Statement 1: q=quotient when dividing by 4, therefore:
y=4q
y^3 = (4q)^3 = 64q^3
64 does not divide by 9 (statement is FALSE), however if q=9 then statement is TRUE. Inconclusive, hence INSUFFICIENT.

Statement 2: r=quotient when dividing by 6:
y=6r
y^3 = (6r)^3 = 216r^3
216 is divisible by 9 (statement is TRUE), therefore y^3 is divisible by 9. SUFFICIENT
Retired Moderator
Joined: 05 Jul 2006
Posts: 1733

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02 Nov 2008, 13:46
1
kman wrote:
If y is an integer, is $$y^3$$ divisible by 9 ?

(1) y is divisible by 4
(2) y is divisible by 6

How can I rephrase the question stem ?

for y^3 to be devisable by 3^2

y has to be in the form 3a

from one .........insuff

from2
y = 3*2*b = 3a..............suff

B
Manager
Joined: 09 Jul 2008
Posts: 107
Location: Dallas, TX
Schools: McCombs 2011

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02 Nov 2008, 13:50
1
OA is B

According to MGMAT, the questions can be rephrased as "Is y divisible by 3 ?"
I don't get how $$y^3$$ divisible by 9 is same as finding "Is y divisible by 3 ?"

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: DS: divisibility &nbs [#permalink] 02 Nov 2008, 13:50
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