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Harish Dorai
 
 

Are x and y both positive? 1) 2x - 2y = 1, 2) x/y > 1

by Harish Dorai Tue Jul 31, 2007 11:21 am

Data sufficiency question:

Are x and y both positive?

1) 2x - 2y = 1

2) x/y > 1

I thought the answer choice is (E), but it is not correct as per GMATPrep software.

My reasoning was as follows:

Statement (1) can be simplified as

x - y = 1/2. This is NOT SUFFICIENT.

Statement (2) can be re-written as x > y. This is NOT SUFFICIENT

Combining the above 2 statements and taking an example as shown below.

x = -3/2 and y = -2. In this case x > y as per the second statement and x - y = 1/2 as per the second statement. Similarly I could also have an example that satisfies x and y as positive. So I went ahead with answer choice (E).

However what I failed to realize in the above example was that it actually doesn't satisfy the original condition x/y > 1. Because -3/2 divided by -2 will give 3/4 which is less than 1.

So is it correct to re-write the inequality x/y > 1 to x > y?
GMAT 2007
 
 

by GMAT 2007 Tue Jul 31, 2007 1:19 pm

Harish another approach: -

I agree (1) or (2) are INSUFFICIENT. but consider both of them together

(1) 2x-2y = 1
(2) x/y >1

Rephrasing (1)

x-y = 1/2
x = y + 1/2

Now substituting value of x in (2)

(y+1/2)/y >1

Solving:

1+ 1/2y > 1

so, 1/2y > 0 it means y >0 since x = y+1/2 s0 X >0 as well.

Hope it helps

GMAT 2007
GMAT 2007
 
 

by GMAT 2007 Tue Jul 31, 2007 1:20 pm

So, the answer should be (C). Am I correct?

GMAT 2007
a7lee
 
 

by a7lee Tue Jul 31, 2007 1:55 pm

The answer is C.

For A) 2x - 2y = 1 -----> x - y = 1/2. You can have 0 - (-1/2) = No or You can have +1 - (+1/2) = Yes. So A is insufficient.

For B) x/y > 1 ---> Either X and Y are both + or X and Y are both negative. So B is insufficent. NOTE: |x| > |y|.

For A+B.

Have x and y be positive and make it work with equation A. So +1 - (+1/2) = 1/2 Yes.
.... be negative and make it work with equation B. So -1 - (-y) = 1/2. y = (3/2) which is > |x|. So you will see that two negatives cannot work because it violtates the rule that x/y > 1. So for A+B the answer is yes.
Harish Dorai
 
 

by Harish Dorai Tue Jul 31, 2007 2:13 pm

Thanks for the explanation. (C) is the right answer.
givemeanid
 
 

by givemeanid Tue Jul 31, 2007 2:16 pm

Statement (2) can be re-written as x > y.


You CANNOT do this. You do not know at this point whether y > 0 or y < 0.
GMAT 2007
 
 

by GMAT 2007 Tue Jul 31, 2007 2:43 pm

givemeanid,

In my approach, with help of (1) & (2) , I have calculated y. Instead of substituting values, the calculation makes it clear that y >0 and so x>0. Hence both are +ve. I feel it was the better approach rather than picking numbers. It helped to stayaway from intricacies of possibility of -ve values.

GMAT 2007
givemeanid
 
 

by givemeanid Tue Jul 31, 2007 5:01 pm

GMAT 2007, your solution is good. I also tend to do the same before using numbers for inequalities!
dbernst
ManhattanGMAT Staff
 
Posts: 301
Joined: Mon May 09, 2005 9:03 am
 

by dbernst Thu Aug 02, 2007 11:31 am

Good discussion all. Now that's teamwork!
unique
 
 

by unique Fri Aug 10, 2007 2:09 pm

I did it this way -

1. 2(x-y) =1
x-y = 1/2 clearly insufficient

2. x/y > 1

x>y when y > 0
x<y when y< 0

insufficient

TOGETHER x-y =1/2 means x>y

From 2 x>y when y>0 so x>0 answer C
Guest
 
 

by Guest Fri Jun 13, 2008 12:13 am

GMAT 2007 wrote:
so, 1/2y > 0 it means y >0


Im sure this is pretty remedial for you guys, but can someone really quickly explain how from 1/2y > 0 = y>0 ??

Thanks!
rfernandez
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Posts: 383
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by rfernandez Fri Jun 13, 2008 4:26 am

Im sure this is pretty remedial for you guys, but can someone really quickly explain how from 1/2y > 0 = y>0 ??


Multiply both sides of the inequality by 2. 2 * 1/2y yields y; 2 * 0 yields 0.
albert.chi
Course Students
 
Posts: 6
Joined: Sun Aug 10, 2008 8:05 am
 

Re: Are x and y both positive? 1) 2x - 2y = 1, 2) x/y > 1

by albert.chi Wed Aug 05, 2009 1:33 am

Can someone help me understand how my 'intuition' is incorrect?

To me:

Statement 1) says that the difference between X and Y is .5, no matter where on the number line they are. Both cases work:
<-------0-------Y(1)---X(1.5)---->
or
<-Y(-2)---X(-1.5)---0------------>

Statement 2) says that X is greater than Y and that they have the same sign (negative or positive)

-
Now taking these together without plugging them into each other like the solution above, isn't all the information I have just saying that:

X is to the right of Y on the number line, they are spaced .5 apart, and they both have the same sign? (Therefore E)

Can someone please help me find the missing piece in my logic to understand why it's C?

Thanks
albert.chi
Course Students
 
Posts: 6
Joined: Sun Aug 10, 2008 8:05 am
 

Re: Are x and y both positive? 1) 2x - 2y = 1, 2) x/y > 1

by albert.chi Sun Aug 09, 2009 7:27 am

Hi, can someone please answer my question in case it was missed?

Thanks!
Ben Ku
ManhattanGMAT Staff
 
Posts: 818
Joined: Sat Nov 03, 2007 7:49 pm
 

Re: Are x and y both positive? 1) 2x - 2y = 1, 2) x/y > 1

by Ben Ku Tue Aug 18, 2009 11:35 pm

Statement 2) says that X is greater than Y and that they have the same sign (negative or positive)


This quote is incorrect. X > Y only if both are positive. For example if x = 3 and y =2, then x/y = 3/2.

However, if both x and y are negative, then x < y. For example, if x = -3 and y = -2, then x / y = 3/2. Here, x < y.

So (2) basically states that x > y > 0 or x < y < 0. (1) states that x > y, so then they must both be positive. Hope that makes sense.
Ben Ku
Instructor
ManhattanGMAT