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If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2

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If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2 [#permalink]

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Re: If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2 [#permalink]

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New post 01 Oct 2015, 13:31
Bunuel wrote:
If x = y^2, what is the value of y – x?

(1) x = 4
(2) x + y = 2

Kudos for a correct solution.


(1) x=4, y=+/-2 Not Suff
(2) x+y=2 -> y^2+y-2=0 than y=-2,1 Not Suff
(1+2) E
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Re: If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2 [#permalink]

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If x = y^2, what is the value of y – x?

(1) x = 4
(2) x + y = 2

Kudos for a correct solution.


st 1 --- x = 4 , given that x = y^2
hence y^2 = 4 , y can be 2 or -2

Hence not sufficient as we are two values of y .

st 2

x + y =2
but given that x = y^2

y^2 + y = 2

y(y + 1) = 2

here also we are getting two different value of y .hence not sufficent

together
x=4
x+y =2

solving them we y = -2

also the value of x =4 and y = -2 --- satisfy x = y^2

hence ans is C
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Re: If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2 [#permalink]

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Bunuel wrote:
If x = y^2, what is the value of y – x?

(1) x = 4
(2) x + y = 2

Kudos for a correct solution.


VERITAS PREP OFFICIAL SOLUTION:

Question Type: What Is the value? This question asks for the value of y – x.

Given information from the question stem: x = y^2. While it may not be important for this question, you should train yourself to notice things like “x cannot be negative” here, as y^2 has to be > 0.

Statement 1: x = 4. This is not sufficient because even taken with the fact above it still results in two answers. Substitute x in the equation from the facts and you get 4 = y^2. So y can equal 2 or – 2. These two values for y will give you different answers for “y – x” and are therefore not specific enough. You need a single value in order to be sufficient and you do not have one. The answer will either be B, C, or E.

Statement 2: x + y = 2. This is not sufficient even if taken together with the information from the question stem. A quick example using Play Devil’s Advocate might be the simplest way to show that this is not sufficient. “x = 1 and y = 1”is compatible with the given information and with Statement 2. However, so is one of our examples from statement 1 “x = 4 and y = -2.” After all 4 + -2 = 2. These different values for x and y will result in different answers to “y – x = ?” This statement is also not sufficient. The answer is either C or E.

Together: Taken together the statements are sufficient. Only one set of values will work with the two statements and the fact. “x = 4 and y = -2.” The correct answer is C. One important takeaway from this question: Do not assume that a variable is positive! When people answer this question incorrectly it’s usually because they only considered 2 as a value (and not -2) for y when they saw Statement 1. When a statement gives you a solution for y^2, remember that y could be either positive or negative. “Forgetting about negatives” is one of the easiest (and most common) mistakes to make on Data Sufficiency problems.
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Re: If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2 [#permalink]

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Re: If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2   [#permalink] 10 Nov 2017, 06:25
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