Are both X and Y positive?
(1) 2x - 2y = 1
(2) x/y > 1
This is from the GMAT prep test. Says the answer is C but I'm not understanding the reasoning. Can anyone help please?
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DS Positives/Negatives
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Statement 1: 2x-2y = 1.Are x and y both positive ?
(1) 2x - 2y = 1
(2) x/y > 1
2(x-y) = 1.
x-y = 1/2.
x = y + 1/2.
It's possible that y=1/2 and x=1.
It's possible that y=0 and x=1/2.
Since in the first case x and y are both positive and in the second case x and y are not both positive, INSUFFICIENT.
Statement 2: x/y > 1.
It's possible that x=2 and y=1, since 2/1 > 1.
It's possible that x=-2 and y=-1, since (-2)/(-1) > 1.
Since in the first case x and y are both positive and in the second case x and y are not both positive, INSUFFICIENT.
Statements 1 and 2 combined:
Statement 1: x = y + 1/2.
Statement 2: x/y > 1.
Substituting for x in the inequality:
(y + 1/2)/y > 1.
1 + 1/(2y) > 1.
1/(2y) > 0.
1/y > 0.
Thus, y>0.
Since y>0 and x = y + 1/2, we know that x>1/2.
Thus, x and y are BOTH POSITIVE.
SUFFICIENT.
The correct answer is C.
First take-away:
The approach above combined two techniques: algebra and plugging in values.
Many DS questions are best solved using a combination of these two techniques.
Second take-away:
Given an equation with 2 variables (such as x = y + 1/2) and an inequality with the same 2 variables (such as x/y > 1), use the equation to substitute for one of the variables in the inequality.
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Hi dunnec3,
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
GMAT assassins aren't born, they're made,
Rich
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
GMAT assassins aren't born, they're made,
Rich
