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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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Math Expert
Joined: 02 Sep 2009
Posts: 50670
If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2  [#permalink]

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01 Oct 2015, 00:49
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If x = y^2, what is the value of y – x?

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

Project DS Butler: Day 4: Data Sufficiency (DS8)

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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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01 Oct 2015, 12:31
1
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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01 Oct 2015, 20:07
2
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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06 Oct 2015, 07:16
1
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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09 Nov 2018, 08:59
gmatbusters wrote:

Project DS Butler: Day 4: Data Sufficiency (DS2)

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

ST 1. x = 4 i.e. 4 = y^2 , but y can be -2 not sufficient

ST 2 x + y = 2 i.e x = y^2 then it means x =1 ; y =1 Sufficient

IMO: B

hopefully I am correct
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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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Updated on: 09 Nov 2018, 09:38
gmatbusters wrote:

Project DS Butler: Day 4: Data Sufficiency (DS2)

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

tricky.

$$x =y^2$$

y - x

$$y - y^2$$

Statement 1: x = 4. $$y^2 = 4$$. y = +2 or - 2 .Not sufficient.

Statement 2:

x + y =2

$$y^2 + y = 2.$$

$$y^2 + y -2 = 0$$

( y + 2 ) ( y - 1) = 0

y + 2 = 0

y = -2

or

y -1 = 0

y = 1.

Not sufficient.

Combining : common value is - 2. Sufficient.

Originally posted by selim on 09 Nov 2018, 09:09.
Last edited by selim on 09 Nov 2018, 09:38, edited 2 times in total.
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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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09 Nov 2018, 09:25
2
Quote:
If $$x = y^2$$, what is the value of $$y - x$$ ?

Quote:
1) $$x = 4$$

$$y^2$$ = 4
Take square root on both side:
|y| = 2
y = 2, -2

Quote:
2) $$x + y = 2$$

$$y^2$$ + y - 2 = 0
(y+2)(y-1) = 0
y = 1, -2

Combining both, y = -2.
OA: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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09 Nov 2018, 09:34
Given: $$x = y^2$$

Stmt 1: x = 4, hence $$4 = y^2$$ or y = +2 or -2, which will give different values for y - x, hence insufficient. Eliminate A, D

Stmt 2: x + y = 2 or y^2 + y - 2 = 0 which gives:
(y+2)*(y-1) = 0 which gives, y = 1 or y = -2. Again, 2 values for y gives 2 values for y - x, hence insufficient. Eliminate B.

Combined: if x = 4, then substitute in x + y = 2 to get, y = -2, hence y - x = -2-4 = -6, hence sufficient. Eliminate E

Hence Option 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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09 Nov 2018, 20:31
gmatbusters wrote:

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

Question: $$y-x = ?$$

Given: $$x = y^2$$

Statement 1: $$x = 4$$
i.e. $$y^2 = 4$$
i.e. $$y = +2$$
i.e. $$y-x = -2-4 or 2-4$$
NOT SUFFICIENT

Statement 2: $$x + y = 2$$
i.e. $$y^2 + y = 2$$
i.e. $$y(y-1) = 2$$
i.e. $$y = 2$$ OR $$y = -1$$
NOT SIFFICIENT

Combining the two statements
$$x + y = 2$$ and $$x=4$$
i.e. $$y = -2$$
SUFFICIENT

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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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09 Nov 2018, 23:31
gmatbusters wrote:

Project DS Butler: Day 4: Data Sufficiency (DS2)

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

Stmnt 1: we get y=+/-2
not sufficient
stmtnt 2: we y(y+1)=2
y=2 and y=1
not sufficient

combining two stmnts:
we get y=-2 , sufficient C
Re: If x = y^2, what is the value of y – x? (1) x = 4 (2) x + y = 2 &nbs [#permalink] 09 Nov 2018, 23:31
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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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