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If m, k, x and y are positive numbers...

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If m, k, x and y are positive numbers...

by A4FEVER » Wed Nov 12, 2008 6:16 pm
is mx + ky > kx + my?

(1) m > k

(2) x > y

Hi all - this is from the GMAC practice test. Can anyone help with regards to the best way to approach this number? Picking numbers seems to be a dawnting task for this particular problem.

Thanks
A4Fever
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by cramya » Wed Nov 12, 2008 6:39 pm
I get A) What's the OA buddy?
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by Retake_GMAT » Wed Nov 12, 2008 6:58 pm
Rearrange the Stem


Is mx + ky > kx + my
mx –my > kx-ky
m(x-y) > k (x-y)

is m >k

St I) m>k , SUFF

St II) InSUFF

Hence A
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by cramya » Wed Nov 12, 2008 7:07 pm
Ratke gmat that was precisely my approach also.

We will wait patiently for the OA....

A4fever,

Just so you know; you could use the spoiler function to post the OA's.
Also please post OA'S using SPOLIER if possible when posting questions.
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by nitin86 » Wed Nov 12, 2008 7:31 pm
Retake_GMAT wrote:Rearrange the Stem


Is mx + ky > kx + my
mx –my > kx-ky
m(x-y) > k (x-y)

is m >k

St I) m>k , SUFF

St II) InSUFF

Hence A
A general rule says, if 1/a > 1/b then

(a) If (a * b) > 0, then multiplying both sides of above term by (a * b) , we get b > a
(b) However, if (a * b) < 0, then multiplying both sides of above term by (a * b), we get b < a

On similar concept, We can't just eliminate (i.e divide) (x-y) from both sides.
It may be (x-y) is negative, and in that case we get different answer.

IMO C
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by logitech » Wed Nov 12, 2008 7:34 pm
m(x-y) > k (x-y)

m(x-y) - k (x-y) > 0

(x-y) (m-k) > 0

both of them needs to be either +, or -

1) INSUF
2) INSUF

1+2) SUF

Hence, C

Any objections ?
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by cramya » Wed Nov 12, 2008 7:59 pm
I think you guys may be right. It could be very well be C)
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by logitech » Wed Nov 12, 2008 8:07 pm
cramya wrote:I think you guys may be right. It could be very well be C)
MAY BE ??? :lol:
LGTCH
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by A4FEVER » Thu Nov 13, 2008 8:22 am
Hey guys, answer is C. And thanks for educating me on the spoiler function. I will use it next time I post a question.
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by A4FEVER » Thu Nov 13, 2008 8:42 am
oh and by the way, I also rephrase the question stem by factoring and got A as an answer so I'm glad to see I wasn't the only one :D
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by wrtau23 » Mon Jan 05, 2009 4:39 pm
Can anyone explain why you can't factor with these types of questions? I factored also, but got D since you can factor as

mx - kx > my - ky
x(m-k) > y(m - k)

I assume the answer is because the factors are critical in answering the question so we should never factor out terms for data sufficiency problems?
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by vittalgmat » Mon Jan 05, 2009 6:17 pm
wrtou
Sure! u can factor that way as well as long as u dont "cancel" the expression within (), u are fine.
U can apply the same logic that Logitech applied to get to C.

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by Ian Stewart » Tue Jan 06, 2009 10:47 am
You certainly can factor here, and it's a very good idea to do so. If you can get the factorization logitech arrived at, then the question is quite quick to answer. What you absolutely *cannot* do is divide both sides of your inequality by, say, (x-y). You need to know if (x-y) is positive or negative; if you don't know whether (x-y) is negative, you don't know if you need to reverse the inequality after you divide by (x-y).

This is the most common trap in GMAT inequalities questions: you will often see slightly complicated inequalities with unknowns, often with fractions, where you are tempted to multiply or divide both sides by an *unknown* (like x, or y, or as above, x-y). To do this, you need to know if the unknown is positive or negative. There are three good questions testing precisely this among the last 20 DS questions in the OG if you want more examples- I don't have the book at hand, but it's easy to pick out the inequality questions with letters and fractions in them.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by vscid » Tue Jan 06, 2009 9:13 pm
logitech wrote:m(x-y) > k (x-y)

m(x-y) - k (x-y) > 0

(x-y) (m-k) > 0

both of them needs to be either +, or -

1) INSUF
2) INSUF

1+2) SUF

Hence, C

Any objections ?
great way of solving :)
The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.
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by santhosh_katkurwar » Mon Dec 18, 2017 12:08 am
wrtau23 wrote:Can anyone explain why you can't factor with these types of questions? I factored also, but got D since you can factor as

mx - kx > my - ky
x(m-k) > y(m - k)

I assume the answer is because the factors are critical in answering the question so we should never factor out terms for data sufficiency problems?
Why cant we cancel (m-k) from both sides here?
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