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GMAT Prep- X& Y both positive?

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GMAT Prep- X& Y both positive?

by nsuen » Sun Jun 12, 2016 11:15 pm
Hi-

Q- Are X and Y both positive?
1) 2X-2Y=1
2)X/Y>1

My thinking process
1) 2(x-y)=1 , x-y = 1/2, then Insufficient because X can be + and Y can be +, but also X can be + and Y can be - so not sufficient
2) x> y Insufficient because X can be + and Y can be +, but also X can be + and Y can be - so not sufficient

1+2, i thought it would be insufficient even combined because for x-y =1/2 and x>y, x and y can still be +, but can also be x= + y=- . For example, x= 0.35 y=-0.15= 0.5. However, the answer is C both statement together are sufficient.Can someone spot what did I do wrong here? Thank in advance.
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by GMATGuruNY » Mon Jun 13, 2016 2:08 am
Check my solution here:
https://www.beatthegmat.com/are-x-and-y- ... 89906.html
nsuen wrote:Hi-
1+2, i thought it would be insufficient even combined because for x-y =1/2 and x>y, x and y can still be +, but can also be x= + y=- . For example, x= 0.35 y=-0.15= 0.5. However, the answer is C both statement together are sufficient.Can someone spot what did I do wrong here? Thank in advance.
x=0.35 and y=-0.15 do not satisfy the constraint in Statement 2 that x/y > 1.
To satisfy statement 2, x and y must have the SAME SIGN.
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by Brent@GMATPrepNow » Mon Jun 13, 2016 5:15 am
Are x and y both positive?
1) 2x - 2y =1
2) x/y >1
Target question: Are x and y both positive?

Statement 1: 2x - 2y = 1
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several pairs of numbers that satisfy this condition. Here are two:
Case a: x = 1 and y = 0.5, in which case x and y are both positive
Case b: x = -0.5 and y = -1, in which case x and y are not both positive
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Aside: For more on this idea of plugging in values when a statement doesn't feel sufficient, you can read my article: https://www.gmatprepnow.com/articles/dat ... lug-values

Statement 2: x/y > 1
This tells us that x/y is positive. This means that either x and y are both positive or x and y are both negative. Here are two possible cases:
Case a: x = 4 and y = 2, in which case x and y are both positive
Case b: x = -4 and y = -2, in which case x and y are not both positive
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2
Statement 1 tells us that 2x - 2y = 1.
Divide both sides by 2 to get: x - y = 1/2
Solve for x to get x = y + 1/2

Now take the statement 2 inequality (x/y > 1) and replace x with y + 1/2 to get:
(y + 1/2)/y > 1
Rewrite as: y/y + (1/2)/y > 1
Simplify: 1 + 1/(2y) > 1
Subtract 1 from both sides: 1/(2y) > 0
If 1/(2y) is positive, then y must be positive.

Statement 2 tells us that either x and y are both positive or x and y are both negative.
Now that we know that y is positive, it must be the case that x and y are both positive
Since we can now answer the target question with certainty, the combined statements are SUFFICIENT

Answer = C

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by [email protected] » Mon Jun 13, 2016 8:45 am
Hi nsuen,

DS questions can often be solved with a variety of tactics. Beyond the "math" approaches that can be used, you can also TEST Values (as Brent's solution showcased) and often use Number Properties to your advantage.

Here, we're asked if X and Y are BOTH positive? This is a YES/NO question.

Fact 1: 2X - 2Y = 1

I'm going to do a little algebra to rewrite this equation....
2X = 1 + 2Y
X = 1/2 + Y

Now we know that X MUST be bigger than Y and we can find some quick values to TEST.
If.....
Y = 0
X = 1/2
Then the answer to the question is NO

Y = 1
X = 1.5
Then the answer to the question is YES
Fact 1 is INSUFFICIENT

Fact 2: X/Y > 1

Using Number Properties, we know that X and Y are either BOTH POSITIVE (YES answer) or BOTH NEGATIVE (NO answer).
Fact 2 is iNSUFFICIENT

Combined, we know....
X = 1/2 + Y
X is GREATER than Y
X/Y > 1
X and Y have the same sign

Since X is greater than Y AND they have the same sign, the ONLY way for X/Y > 1 to be possible is when they're BOTH POSITIVE. If they were both negative, then Y would be greater than X....and we know that that's not possible). The answer to the question is ALWAYS YES.
Combined, SUFFICIENT

Final Answer: C

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