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# If xy = -6 , what is the value of xy(x+y ) ?

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Manager
Joined: 17 Nov 2009
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If xy = -6 , what is the value of xy(x+y ) ? [#permalink]

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07 Oct 2010, 06:46
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65% (hard)

Question Stats:

53% (02:38) correct 47% (01:32) wrong based on 163 sessions

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If xy = -6 , what is the value of xy(x+y ) ?

(1) x – y = 5
(2) x*y^2 = 18

OPEN DISCUSSION OF THIS QUESTION IS HERE: if-xy-6-what-is-the-value-of-xy-x-y-167646.html
[Reveal] Spoiler: OA
Manager
Joined: 06 Aug 2010
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Location: Boston
Re: If xy = -6 , what is the value of xy(x+y ) ? [#permalink]

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07 Oct 2010, 06:56
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KUDOS
agnok wrote:
If xy = -6 , what is the value of xy(x+y ) ?
(1) x – y = 5
(2) x*y square = 18

We're given $$xy = -6$$, so $$xy(x+y) = -6(x+y)$$.

(1) x - y = 5 -> x = 5+y, so we have:

$$(5+y)(y) = -6$$
$$5y + y^2 = -6$$
$$y^2 + 5y + 6 = 0$$
$$y = -2$$ or $$y = -3$$.

Two solutions for y - insufficient.

(2) $$x(y^2) = 18$$. We know that $$y = \frac{-6}{x}$$, so we have $$x(\frac{-6}{x^2}) = 18$$. So:

$$x(\frac{36}{x^2}) = 18$$
$$\frac{36}{x} = 18$$
$$x = 2$$

Then y = -3, and $$xy(x+y) = -6(2-3) = 6$$. Sufficient.

B

Last edited by TehJay on 07 Oct 2010, 07:02, edited 1 time in total.
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Posts: 39661
Re: If xy = -6 , what is the value of xy(x+y ) ? [#permalink]

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07 Oct 2010, 07:01
1
KUDOS
Expert's post
agnok wrote:
If xy = -6 , what is the value of xy(x+y ) ?
(1) x – y = 5
(2) x*y square = 18

If xy = -6 , what is the value of xy(x+y ) ?

Given: $$xy=-6$$.
Question: $$xy(x+y)=-6(x+y)=?$$

(1) $$x-y=5$$ --> $$x=y+5$$ --> $$xy=(y+5)y=-6$$ --> $$y^2+5y+6=0$$ --> either $$y=-3$$ and $$x=y+5=2$$ so $$xy(x+y)=-6(x+y)=6$$ OR $$y=-2$$ and $$x=y+5=3$$ so $$xy(x+y)=-6(x+y)=-6$$. Two different answers, hence not sufficient.

(2) $$x*y^2=18$$ --> $$xy*y=-6*y=18$$ --> $$y=-3$$ --> $$x=2$$ --> $$xy(x+y)=-6(x+y)=6$$. Sufficient.

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Re: If xy = -6 , what is the value of xy(x+y ) ? [#permalink]

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08 Oct 2010, 11:14
1
KUDOS
ans is B ...well in GMAT maths ques are not tough bt tricky..mainly in DS
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Re: If xy = -6 , what is the value of xy(x+y ) ? [#permalink]

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09 Jun 2015, 08:31
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: If xy = -6 , what is the value of xy(x+y ) ? [#permalink]

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09 Jun 2015, 08:44
OPEN DISCUSSION OF THIS QUESTION IS HERE: if-xy-6-what-is-the-value-of-xy-x-y-167646.html
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Re: If xy = -6 , what is the value of xy(x+y ) ?   [#permalink] 09 Jun 2015, 08:44
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