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ccassel: Master | Next Rank: 500 Posts; Posts: 118; Joined: Wed Mar 16, 2011 1:44 pm; Location: Canada; Followed by:2 members; GMAT Score:530

DS - value - gmat prep

by ccassel » Wed Apr 27, 2011 10:33 am

I am overlooking something obvious here; can anyone explain the answer?

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

1. wx^2 = 16
2. y = 4

Answer is A

Cheers,

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Source: Beat The GMAT — Data Sufficiency | Wed Apr 27, 2011 10:33 am

manpsingh87: Master | Next Rank: 500 Posts; Posts: 436; Joined: Tue Feb 08, 2011 3:07 am; Thanked: 72 times; Followed by:6 members

by manpsingh87 » Wed Apr 27, 2011 10:38 am

ccassel wrote:I am overlooking something obvious here; can anyone explain the answer?

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

1. wx^2 = 16
2. y = 4

Answer is A

Cheers,

wx=y; xy??

1) wx^2= 16
wx^2 can be written as wx(x); also from question we know that wx=y;
hence yx=16 therefore one is sufficient.

2)y=4;

wx=4; w=1 x=4; therfore xy can be 16;
if w=x=2 then xy will be 2*4=8;
hence 2 alone is not sufficient to answer the question
hence A

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ccassel: Master | Next Rank: 500 Posts; Posts: 118; Joined: Wed Mar 16, 2011 1:44 pm; Location: Canada; Followed by:2 members; GMAT Score:530

by ccassel » Wed Apr 27, 2011 11:12 am

Due to fatigue, I jumped to the conclusion that wx^2= 16 = y^2 when in fact it is not equal.

Thanks for pointing out my mistake manpsingh87.

Cheers,

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pm; Location: New York, NY; Thanked: 13060 times; Followed by:1906 members; GMAT Score:790

by GMATGuruNY » Thu Apr 28, 2011 2:13 am

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

(1) w(x^2) = 16

(2) y = 4

An efficient approach -- one that doesn't require any algebraic insight -- would be to plug in two sets of values that satisfy the conditions given. If the value of xy stays the same, then the statement is sufficient. If the value of xy changes, then the statement is insufficient.

Statement 1: w(x^2) = 16
Let w=4, x=2, so that 4*(2^2) = 16.
Then y = wx = 4*2 = 8.
xy = 2*8 = 16.

Let w=16, x=1, so that 16*(1^2) = 16.
Then y = wx = 16*1 = 16.
xy = 1*16 = 16.

Since the value of xy stays the same, sufficient.

Statement 2: y=4
No way to determine the value of x.
Insufficient.

The correct answer is A.

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