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If x and y are positive numbers and z = xy^2, a 50 percent increase

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If x and y are positive numbers and z = xy^2, a 50 percent increase  [#permalink]

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New post 09 Nov 2015, 00:00
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If x and y are positive numbers and z = xy^2, a 50 percent increase in x and a 20 percent decrease in y would result in which of the following changes in z?

(A) A decrease of 4%
(B) A decrease of 14%
(C) An increase of 4%
(D) An increase of 20%
(E) An increase of 30%

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Re: If x and y are positive numbers and z = xy^2, a 50 percent increase  [#permalink]

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New post 09 Nov 2015, 03:59
Bunuel wrote:
If x and y are positive numbers and z = xy^2, a 50 percent increase in x and a 20 percent decrease in y would result in which of the following changes in z?

(A) A decrease of 4%
(B) A decrease of 14%
(C) An increase of 4%
(D) An increase of 20%
(E) An increase of 30%


z=x*y^2
Z=1.5 x*(0.8)^2=.96x*y^2=.96z
so a decrease of 4%
A
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Re: If x and y are positive numbers and z = xy^2, a 50 percent increase  [#permalink]

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New post 10 Nov 2015, 07:57
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z= x*y^2

Now , since x increases by 50 % and y decreases by 20 %
z'=(3/2)x * (8/10)(8/10)y^2
= 192/200 * x*y^2
= 96/100 * x*y^2
= 96/100 * z

a decrease in 4 %

Answer A
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Re: If x and y are positive numbers and z = xy^2, a 50 percent increase  [#permalink]

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New post 17 Apr 2016, 05:19
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Bunuel wrote:
If x and y are positive numbers and z = xy^2, a 50 percent increase in x and a 20 percent decrease in y would result in which of the following changes in z?

(A) A decrease of 4%
(B) A decrease of 14%
(C) An increase of 4%
(D) An increase of 20%
(E) An increase of 30%


Just take smart numbers
x=4 and y=5-----> z=100
a 50 percent increase in x ---->x=6
a 20 percent decrease in y----->y=4
now z=6*4^2---->96
so Z decreased from 100 to 96
Ans A
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Re: If x and y are positive numbers and z = xy^2, a 50 percent increase  [#permalink]

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New post 30 Apr 2016, 14:09
Given fact z = \(xy^2\)
A 50% increase in x = 1.5x
A 20% decrease in y = 0.8y
calculating z = \(xy^2\)
change in z = \(1.5x\) * \(0.8y^2\)
so z = 0.96 times original z
clearly its a 4 % decrease
correct answer - A
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Re: If x and y are positive numbers and z = xy^2, a 50 percent increase  [#permalink]

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New post 07 Oct 2018, 08:30
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Re: If x and y are positive numbers and z = xy^2, a 50 percent increase &nbs [#permalink] 07 Oct 2018, 08:30
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