If wx = y, then what is the value of xy?

Question

(1) w*x^2 = 16
(2) y = 4

Bunuel · Accepted Answer

If wx=y, then what is the value of xy?

(1) wx^2 = 16 -->  x*(wx)=16  --> as given that  wx=y  then  x*y=16 . Sufficient.

(2) y = 4 --> clearly insufficient: question becomes  4x=? , and all we know is that  wx=4 .

Answer: A.

jonblazon · Answer

super thanks! should have seen it was that easy

MICKEYXITIN · Answer

+1 for Jonblazon for good question. I got C for uncareful observation +1 for Bunuel: you are amazing Thanks once again for your good question, at least to me.

jumli · Answer

Usually when I have a DS question I try to come up with the equations and respective variables that I need. Then I count the equations and unknowns and if I have more unknowns than I have equations I know it's not sufficient.
Here I'm not sure if the approach is feasible, but I'd like to give it a try. Please give me your opinions on the approach:

1. We have the equation wx = y and we need to find xy = ?. In order to do so we don't need to know x or y, but only w (if we have w, we're able to find xy, as wx = y is given). Therefor w is our only unknown.

(1) we get wx^2=16. As we don't need to handle x as an unknown (see above) we can count what we have: here it's 1 unknown (w) and 1 equation => sufficient
(2) we get y=4. y isn't an unknown in this approach, so when we count: 1 unknown (w) and 0 equations => not sufficient

So it's A.

EMPOWERgmatRichC · Answer

Hi All,

Most of the DS questions that you'll see on Test Day will be based on patterns of some kind - even if you don't recognize that a pattern is there. As such, sometimes the way to get to the correct answer is to do enough work to PROVE the pattern by TESTing VALUES.

Here, we're told that (W)(X) = Y and we're asked for the value of (X)(Y).

Fact 1: W(X^2) = 16

IF...
W = 16
X = 1
Y = 16
(X)(Y) = 16

W = 16
X = -1
Y = -16
(X)(Y) = 16

W = 4
X = 2
Y = 8
(X)(Y) = 16

W = 4
X = -2
Y = -8
(X)(Y) = 16

W = 1
X = 4
Y = 4
(X)(Y) = 16

W = 1
X = -4
Y = -4
(X)(Y) = 16

The pattern here seems to prove that XY is ALWAYS equal to 16.
Fact 1 is SUFFICIENT

Fact 2: Y = 4

This tells us that...
(W)(X) = 4

IF...
X = 1
(X)(Y) = 4

X = 4
(X)(Y) = 16
Fact 2 is INSUFFICIENT

Final Answer:  A

GMAT assassins aren't born, they're made,
Rich

mvictor · Answer

i mistakenly took it as (wx)^2
and got wx = +-4 which is insufficient; 2 is as well insufficient..
eh..can't say it's not my fault, but the topic starter should have put the math code here :D

Bunuel · Answer

The point is that wx^2 mathematically can only mean w*x^2. If it were (wx)^2 it would be written that way. Still edited to avoid confusion.

BrentGMATPrepNow · Answer

Given :  wx = y

Target question :    What is the value of xy ?

Statement 1 : wx²= 16  
We can rewrite this as: (wx)(x) = 16
Since  wx = y , we can replace  wx  with  y  to get: ( y )(x) = 16
So, the answer to the target question is  xy = 16 
Since we can answer the  target question  with certainty, statement 1 is SUFFICIENT

Statement 2 : y = 4 
There are several values of w, x and y that satisfy statement 2. Here are two: 
 Case a : w = 1, x = 4 and y = 4. In this case, the answer to the target question is  xy = 16 
 Case b : w = 2, x = 2 and y = 4. In this case, the answer to the target question is  xy = 8 
Since we cannot answer the  target question  with certainty, statement 2 is NOT SUFFICIENT

Answer: A

Cheers,
Brent

Kinshook · Answer

Asked: If wx = y, then what is the value of xy?
xy = x*wx = wx^2

(1) w*x^2 = 16
xy = w*x^2 = 16
SUFFICIENT

(2) y = 4
Since x is unknown
NOT SUFFICIENT

IMO A

arbazfatmi1994 · Answer

Do we have to check for positives and negatives in such questions? Is there a rule? I have doubts about such questions...

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If wx = y, then what is the value of xy?

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GMAT Data Sufficiency (DS) Questions

If wx = y, then what is the value of xy?

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