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# The sides of rectangle X are each multiplied by a to form re

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Senior Manager
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The sides of rectangle X are each multiplied by a to form re [#permalink]

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27 Feb 2014, 12:50
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55% (hard)

Question Stats:

68% (01:55) correct 32% (01:43) wrong based on 280 sessions

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The sides of rectangle X are each multiplied by a to form rectangle Y and by b to form rectangle Z. a times the area of X is 10, and b times the area of X is 5. If the difference in area between Y and Z is 300, what is a - b?

A. 5
B. 20
C. 30
D. 50
E. 60

[Reveal] Spoiler:
Set sides of rectangle X are L and W
Area of X: = LW Area of Y = (aL)(aW) = a^2 * LW Area of Z = (bL)(bW) = b^2 * LW a times area X = aLW = 10 b times area X = bLW = 5
Which means that difference between Y and Z = aaLW – bbLW = 300.
And since you're looking to solve for a - b, you can try to get a and b alone by factoring out the common LW terms:
LW(a^2 - b^2) = 300
Which gives you the Difference of Squares setup that allows you to get (a - b) alone:
LW(a + b)(a - b) = 300
Then you should see that if you distribute LW across the first set of parentheses, you can get aLW and bLW, for which you have actual values:
(aLW + bLW)(a - b) = 300
(10 + 5)(a - b) = 300 15(a - b) = 300 (a - b) = 20

Hi, I want to know if we have the simpler solution, please
[Reveal] Spoiler: OA
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Joined: 16 Oct 2010
Posts: 7938
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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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27 Feb 2014, 20:35
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goodyear2013 wrote:
The sides of rectangle X are each multiplied by a to form rectangle Y and by b to form rectangle Z. a times the area of X is 10, and b times the area of X is 5. If the difference in area between Y and Z is 300, what is a - b?
5
20
30
50
60

[Reveal] Spoiler:
Set sides of rectangle X are L and W
Area of X: = LW Area of Y = (aL)(aW) = a^2 * LW Area of Z = (bL)(bW) = b^2 * LW a times area X = aLW = 10 b times area X = bLW = 5
Which means that difference between Y and Z = aaLW – bbLW = 300.
And since you're looking to solve for a - b, you can try to get a and b alone by factoring out the common LW terms:
LW(a^2 - b^2) = 300
Which gives you the Difference of Squares setup that allows you to get (a - b) alone:
LW(a + b)(a - b) = 300
Then you should see that if you distribute LW across the first set of parentheses, you can get aLW and bLW, for which you have actual values:
(aLW + bLW)(a - b) = 300
(10 + 5)(a - b) = 300 15(a - b) = 300 (a - b) = 20

Hi, I want to know if we have the simpler solution, please

Assume the side of rectangle X is s.

X (side s, area s^2)
Y (side as, area (as)^2)
Z (side bs, area (bs)^2)

Given: $$as^2 = 10$$, $$bs^2 = 5$$ ..........(I)
$$(as)^2 - (bs)^2 = 300$$
$$s^2 (a+b)(a-b) = 300$$ ...........(II)

We need to find (a-b).
From (I), $$as^2 + bs^2 = 15 = s^2(a+b)$$ (you do this because you need to get rid of s^2 and (a+b) in equation (II) above)
Substitute this in (II) to get $$15*(a-b) = 300$$
(a-b) = 20
_________________

Karishma
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Intern Status: Taking GMAT Joined: 30 Sep 2013 Posts: 4 Location: United States Concentration: Operations, International Business Schools: Kellogg '17 GMAT Date: 06-15-2014 GPA: 3.2 WE: Analyst (Consulting) Re: The sides of rectangle X are each multiplied by a to form re [#permalink] ### Show Tags 27 Feb 2014, 23:02 2 This post received KUDOS 2 This post was BOOKMARKED VeritasPrepKarishma wrote: goodyear2013 wrote: The sides of rectangle X are each multiplied by a to form rectangle Y and by b to form rectangle Z. a times the area of X is 10, and b times the area of X is 5. If the difference in area between Y and Z is 300, what is a - b? 5 20 30 50 60 [Reveal] Spoiler: Set sides of rectangle X are L and W Area of X: = LW Area of Y = (aL)(aW) = a^2 * LW Area of Z = (bL)(bW) = b^2 * LW a times area X = aLW = 10 b times area X = bLW = 5 Which means that difference between Y and Z = aaLW – bbLW = 300. And since you're looking to solve for a - b, you can try to get a and b alone by factoring out the common LW terms: LW(a^2 - b^2) = 300 Which gives you the Difference of Squares setup that allows you to get (a - b) alone: LW(a + b)(a - b) = 300 Then you should see that if you distribute LW across the first set of parentheses, you can get aLW and bLW, for which you have actual values: (aLW + bLW)(a - b) = 300 (10 + 5)(a - b) = 300 15(a - b) = 300 (a - b) = 20 Hi, I want to know if we have the simpler solution, please Assume the side of rectangle X is s. X (side s, area s^2) Y (side as, area (as)^2) Z (side bs, area (bs)^2) Given: $$as^2 = 10$$, $$bs^2 = 5$$ ..........(I) $$(as)^2 - (bs)^2 = 300$$ $$s^2 (a+b)(a-b) = 300$$ ...........(II) We need to find (a-b). From (I), $$as^2 + bs^2 = 15 = s^2(a+b)$$ (you do this because you need to get rid of s^2 and (a+b) in equation (II) above) Substitute this in (II) to get $$15*(a-b) = 300$$ (a-b) = 20 Did you consider a rectangle as a square? Because if it is a rectangle then the sides may or may not be equal. Please clarify. SVP Status: The Best Or Nothing Joined: 27 Dec 2012 Posts: 1839 Location: India Concentration: General Management, Technology WE: Information Technology (Computer Software) Re: The sides of rectangle X are each multiplied by a to form re [#permalink] ### Show Tags 28 Feb 2014, 01:23 Every square is a rectangle however every rectangle need not be a square. Considering the same, the answer comes up correct = 20 = Answer = B _________________ Kindly press "+1 Kudos" to appreciate Current Student Joined: 25 Sep 2012 Posts: 281 Location: India Concentration: Strategy, Marketing GMAT 1: 660 Q49 V31 GMAT 2: 680 Q48 V34 Re: The sides of rectangle X are each multiplied by a to form re [#permalink] ### Show Tags 28 Feb 2014, 06:56 it doesn't matter here if its a square o rectanlge value gets replaced later. take it as s^2 or a*b Intern Status: Taking GMAT Joined: 30 Sep 2013 Posts: 4 Location: United States Concentration: Operations, International Business Schools: Kellogg '17 GMAT Date: 06-15-2014 GPA: 3.2 WE: Analyst (Consulting) Re: The sides of rectangle X are each multiplied by a to form re [#permalink] ### Show Tags 28 Feb 2014, 09:54 b2bt wrote: it doesn't matter here if its a square o rectanlge value gets replaced later. take it as s^2 or a*b Agreed, it gets cancelled, sorry for the trouble Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7938 Location: Pune, India Re: The sides of rectangle X are each multiplied by a to form re [#permalink] ### Show Tags 01 Mar 2014, 18:41 1 This post received KUDOS Expert's post Fantabulous wrote: Did you consider a rectangle as a square? Because if it is a rectangle then the sides may or may not be equal. Please clarify. I have assumed that the rectangle is a square since square is also a type of rectangle (but I see I missed writing it). It is a PS question so it will have a unique answer. This means a - b will be the same for every such group of rectangles. So we can easily take one such group such that the rectangles are squares to make our calculations easier. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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21 Mar 2015, 00:01
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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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27 Apr 2015, 18:32
Could you please let me know why you do not distribute the s^2 through the parenthesis? I asked this in my own topic but it has since been locked and no further information was provided to me except to link to this thread.

$$s^2(a+b)(a−b)=300$$ ...........(II)

Why does this not simplify to:
$$15∗s^2a−s^2b)$$

veritas-prep-the-sides-of-rectangle-x-are-each-multiplied-by-a-to-for-196927.html
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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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27 Apr 2015, 19:30
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willowtree2 wrote:
Could you please let me know why you do not distribute the s^2 through the parenthesis? I asked this in my own topic but it has since been locked and no further information was provided to me except to link to this thread.

$$s^2(a+b)(a−b)=300$$ ...........(II)

Why does this not simplify to:
$$15∗s^2a−s^2b)$$

veritas-prep-the-sides-of-rectangle-x-are-each-multiplied-by-a-to-for-196927.html

$$(a+b)*(a-b) = a^2 - b^2$$
You can simplify this to $$s^2a^2 - s^2b^2 = 300$$ but how will that take you to the answer? You need the value of $$(a - b)$$ so you need to get rid of only $$s^2(a+b)$$.
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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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05 Jun 2016, 11:37
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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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03 Sep 2017, 20:30
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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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04 Sep 2017, 02:57
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Let rectangle X length=l width=w => Area = l*w
So now: rectangle Y length=a*l width=a*w => Area =a^2* l*w
rectangle Z length=b*l width=b*w => Area =b^2* l*w

given: a* Area of X = 10 = a*l*w
b* Area of X = 5 = b*l*w

so we can write a*l*w+b*l*w=10+5
=> l*w(a+b)=15

Also given => Area of Y - area of Z = 300 = a^2*l*w -b^2*l*w

so lw(a^2-b^2)=300
=> l*w(a+b)(a-b)=300
=> 15 (a-b)=300
=> (a-b)= 20

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Re: The sides of rectangle X are each multiplied by a to form re [#permalink]

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14 Nov 2017, 04:11
VeritasPrepKarishma wrote:
goodyear2013 wrote:
The sides of rectangle X are each multiplied by a to form rectangle Y and by b to form rectangle Z. a times the area of X is 10, and b times the area of X is 5. If the difference in area between Y and Z is 300, what is a - b?
5
20
30
50
60

[Reveal] Spoiler:
Set sides of rectangle X are L and W
Area of X: = LW Area of Y = (aL)(aW) = a^2 * LW Area of Z = (bL)(bW) = b^2 * LW a times area X = aLW = 10 b times area X = bLW = 5
Which means that difference between Y and Z = aaLW – bbLW = 300.
And since you're looking to solve for a - b, you can try to get a and b alone by factoring out the common LW terms:
LW(a^2 - b^2) = 300
Which gives you the Difference of Squares setup that allows you to get (a - b) alone:
LW(a + b)(a - b) = 300
Then you should see that if you distribute LW across the first set of parentheses, you can get aLW and bLW, for which you have actual values:
(aLW + bLW)(a - b) = 300
(10 + 5)(a - b) = 300 15(a - b) = 300 (a - b) = 20

Hi, I want to know if we have the simpler solution, please

Assume the side of rectangle X is s.

X (side s, area s^2)
Y (side as, area (as)^2)
Z (side bs, area (bs)^2)

Given: $$as^2 = 10$$, $$bs^2 = 5$$ ..........(I)
$$(as)^2 - (bs)^2 = 300$$
$$s^2 (a+b)(a-b) = 300$$ ...........(II)

We need to find (a-b).
From (I), $$as^2 + bs^2 = 15 = s^2(a+b)$$ (you do this because you need to get rid of s^2 and (a+b) in equation (II) above)
Substitute this in (II) to get $$15*(a-b) = 300$$
(a-b) = 20

Another way of doing this..

Let the original sides be x and y ---- area = $$xy$$

For Rectangle X -- ax and ay ---- area = $$a^2xy$$

For Rectangle Y -- bx an by ---- area = $$b^2xy$$

Now, it's given that

$$axy = 10$$
Thus
$$a = \frac{10}{(xy)}$$

Similarly we can write for Z as

$$bxy = 5$$
Thus
$$b = \frac{5}{(xy)}$$

It can clearly be seen that

$$a = 2b$$

Substituting "axy" in the original area equation..we can see that the area of X can be written as = 10a
Similarly for B it can be written as = 5b

Difference in areas

$$10a - 5b = 300$$

Using value of a as 2b, we get

b = 20
a = 40
Thus
a-b = 20
(B)
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Re: The sides of rectangle X are each multiplied by a to form re   [#permalink] 14 Nov 2017, 04:11
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