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0 < xy [#permalink] New post 07 Jun 2017, 01:18
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Question Stats:

58% (01:41) correct 41% (01:43) wrong based on 39 sessions




\(0 < xy\)

Quantity A
Quantity B
\(\frac{5}{2x} - \frac{2}{2y}\)
\(\frac{5y-2x}{2x-2y}\)


A) Quantity A is greater.
B) Quantity B is greater.
C) The two quantities are equal.
D) The relationship cannot be determined from the information given.
[Reveal] Spoiler: OA

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Re: 0 < xy [#permalink] New post 09 Jun 2017, 10:37
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Expert's post
Carcass wrote:




\(0 < xy\)

Quantity A
Quantity B
\(\frac{5}{2x} - \frac{2}{2y}\)
\(\frac{5y-2x}{2x-2y}\)




Given:
Quantity A: 5/2x - 2/2y
Quantity B: (5y-2x)/(2x-2y)

Let's see what happens if we rewrite Quantity A as one big fraction.
First get common denominators...
Quantity A: 5y/2xy - 2x/2yx
Quantity B: (5y-2x)/(2x-2y)

Combine to get:
Quantity A: (5y-2x)/2yx
Quantity B: (5y-2x)/(2x-2y)

NICE!! This numerators are the same for both fractions.
This gives me the opportunity to use a strategy I call "Looking for Equality" (see our video below for more on this)
I can see that if I choose an x- and y-value that makes the numerators both zero, this will speed up my solution. You'll see why shortly.

I can see that, if x = 5 and y = 2, then we can plug these values into the quantities to get:
Quantity A: [5(2)-2(5)]/(2)(2)(5)
Quantity B: [5(2)-2(5)]/[2(5)-2(2)]

Evaluate:
Quantity A: 0
Quantity B: 0

So, when x = 5 and y = 2 the quantities are EQUAL.
The correct answer is EITHER C or D

At this point, every pair of x- and y-values will either yield equal quantities or not.
Let's plug in super-easy numbers.
We'll plug in x = 1 and y = 1
We get:
Quantity A: [5(1)-2(1)]/(2)(1)(1)
Quantity B: [5(1)-2(1)]/[2(1)-2(1)]

Evaluate:
Quantity A: 3/2
Quantity B: 3/0
These quantities are definitely NOT EQUAL.

Answer:
[Reveal] Spoiler:
D


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Re: 0 < xy [#permalink] New post 30 Jan 2018, 10:47
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From the given condition, x and y have to both positive or negative.
Now putting value x=1, y=1
Q.A 3/2 Q.B undefined
Now putting x=1,y=2
Q.A 2 Q.B -4 , Q.A>Q.B
Now putting x=-1,y=-1
Q.A -3/2 Q.B undefined

So the relation can't be determined.
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Re: 0 < xy [#permalink] New post 18 Feb 2018, 06:43
I've tried for x=1, y=2, x=1, y=3, and x=-1, y=-2, and getting A as the answer. What to do now? How can I chose sensible numbers?
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Re: 0 < xy [#permalink] New post 19 Feb 2018, 02:35
Expert's post
gremather wrote:
I've tried for x=1, y=2, x=1, y=3, and x=-1, y=-2, and getting A as the answer. What to do now? How can I chose sensible numbers?


-2,-1,0,1,2 these are good numers to pick.

Of course, if the question does not specify whether they are integers -1/2, 1/2 should be included in your trial and error test.

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Re: 0 < xy [#permalink] New post 01 Mar 2018, 11:59
It was a good one.
Answer is Choice D.
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Last edited by GREMasterBlaster on 04 Mar 2018, 01:40, edited 1 time in total.
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Re: 0 < xy [#permalink] New post 04 Mar 2018, 00:54
i guess this is a good one
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Re: 0 < xy   [#permalink] 04 Mar 2018, 00:54
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