Does 4x+5y = 0?

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bharathh: Master | Next Rank: 500 Posts; Posts: 159; Joined: Thu Aug 27, 2009 10:30 am; Thanked: 19 times

by bharathh » Sun Sep 06, 2009 7:23 pm

DS question from the Princeton GMAT practice test. Didn't understand the explanation. Anyone else have any ideas?

Does 4x+5y = 0?

(1) y=0
(2) 8x=-10y

OA: Answer is B. I chose D

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Source: Beat The GMAT — Data Sufficiency | Sun Sep 06, 2009 7:23 pm

Bryant@VeritasPrep: MBA Admissions Consultant; Posts: 1090; Joined: Wed May 27, 2009 4:06 amFollowed by: 68 members; Thanked: 175 times; GMAT Score:750

by Bryant@VeritasPrep » Mon Sep 07, 2009 7:04 pm

the second equation alone is sufficient along with the given equation to prove that 4x + 5y = 0, since you would have two equations and two unknowns and sovling them results in a valid answer
( -5y + 5y = 0). Not only do you not need to know that y=0, y does not have to be zero to solve the two equations.

Bryant Michaels
MBA Admissions Consultant

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bharathh: Master | Next Rank: 500 Posts; Posts: 159; Joined: Thu Aug 27, 2009 10:30 am; Thanked: 19 times

by bharathh » Tue Sep 08, 2009 8:48 am

Thanks Bryant... but I guess I did not make my question clearer. Why is A not sufficient by itself? I can understand why B is sufficient.

If I look at the given information if y=0, x=0.. So the eqn would equal 0. Why is that insufficient?

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alex10: Junior | Next Rank: 30 Posts; Posts: 24; Joined: Fri Aug 21, 2009 9:35 am

by alex10 » Tue Sep 08, 2009 9:26 am

A: y = 0

Therefore 4x = 0

x COULD be 0, then 4x = 0, yes. But x could be any other number as well. 2,3,12,66. 4 times 66 certainly doesn't equal 0. We don't have any information regarding the value of x. So insufficient.

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gmat579: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Mon Sep 15, 2008 5:14 pm; Thanked: 2 times; GMAT Score:740

by gmat579 » Tue Sep 08, 2009 9:31 am

bharathh wrote:Thanks Bryant... but I guess I did not make my question clearer. Why is A not sufficient by itself? I can understand why B is sufficient.

If I look at the given information if y=0, x=0.. So the eqn would equal 0. Why is that insufficient?

Bharath, it looks like you assumed that 4x+5y = 0 and then solved for x when y = 0. Actually the questions asks you to for sufficiency to prove whether 4x+5y = 0.
From Stmt 1 - Given Y = 0, we still dont know the value for X and hence teh value of the equation is not known, so 1 is not sufficient.

Stmt 2 - on rearraging the equation in 2 we get 4x + 5y = 0, so sufficient.

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bharathh: Master | Next Rank: 500 Posts; Posts: 159; Joined: Thu Aug 27, 2009 10:30 am; Thanked: 19 times

by bharathh » Tue Sep 08, 2009 9:35 am

Ah! Finally the tubelight flickers to life.

I get it. Thanks all!

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