Which of these two is the greater, x or y?

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rs2010: Master | Next Rank: 500 Posts; Posts: 232; Joined: Fri Jul 04, 2008 4:14 pmFollowed by: 1 member; Thanked: 14 times; GMAT Score:760

by rs2010 » Thu Jul 10, 2008 12:51 pm

23. Which of these two is the greater, x or y?
x – y less than 0
x + y greater than 0
A. 1 alone, not 2 alone
B. 2 alone, not 1 alone
C. 1 and 2 together (need both) Answer
D. 1 alone or 2 alone
E. 1 and 2 together are not sufficient

I selected A but OA is E.

From "How to pass the GMAT" by Mike Bryon.

he he template was not allowing me to add sings

One more

Which of these two is the greater, x or y?
(1) 1/x > 1/y
(2) Both are negative
A. 1 alone, not 2 alone
B. 2 alone, not 1 alone
C. 1 and 2 together (need both) Answer
D. 1 alone or 2 alone
E. 1 and 2 together are not sufficient

OA is C, I selected A

Last edited by rs2010 on Thu Jul 10, 2008 1:45 pm, edited 2 times in total.

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Source: Beat The GMAT — Data Sufficiency | Thu Jul 10, 2008 12:51 pm

target790: Junior | Next Rank: 30 Posts; Posts: 24; Joined: Sun May 04, 2008 12:18 pm; Thanked: 1 times

by target790 » Thu Jul 10, 2008 1:32 pm

First question has some typo.

2nd question>You considered (a) just thinking of positive values.Consider negative values ,you will come down to C immediately.It's only the combination of both which leads to valid conclusion.

Cheers

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rs2010: Master | Next Rank: 500 Posts; Posts: 232; Joined: Fri Jul 04, 2008 4:14 pmFollowed by: 1 member; Thanked: 14 times; GMAT Score:760

by rs2010 » Thu Jul 10, 2008 1:48 pm

I am deducing 1/x > 1/y to y > x.

What's wrong with that ?

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target790: Junior | Next Rank: 30 Posts; Posts: 24; Joined: Sun May 04, 2008 12:18 pm; Thanked: 1 times

by target790 » Thu Jul 10, 2008 2:22 pm

nothing wrong if you are considering both the numbers positive or both negative.Only when the signs are different we need to be careful.

say:

1/x>1/y =>y>x (only if signs are identical)
1/x>1/y =>y<x(if signs are different)

Hope I'm clear this time.

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GMAT/MBA Expert

Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Thu Jul 10, 2008 2:33 pm

hemantsood wrote:I am deducing 1/x > 1/y to y > x.

What's wrong with that ?

You are multiplying both sides of the inequality by xy. If xy is negative, you would need to reverse the inequality. If xy is positive, then your conclusion is valid.

And for this question:

23. Which of these two is the greater, x or y?
x – y less than 0
x + y greater than 0

if x-y < 0, then certainly x < y (add y to both sides). 1) is certainly sufficient here. 2) is clearly insufficient, so the answer should be A.

I'm not familiar with the source, so cannot comment on whether it's worthwhile to be using it for study, but if it contains a lot of mistaken questions like this one, I'd suggest you look elsewhere. When I say that the question is in error, I'm assuming you've transcribed the question and OA correctly.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

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rs2010: Master | Next Rank: 500 Posts; Posts: 232; Joined: Fri Jul 04, 2008 4:14 pmFollowed by: 1 member; Thanked: 14 times; GMAT Score:760

by rs2010 » Thu Jul 10, 2008 3:58 pm

Thanks Target790 believe me I did not want to write this name.

I was not condisering the case with diffrent signs.

Thanks Ian too.
I too realized later after finding so many mistakes in the book.
Initially I thought may be I am wrong so I asked couple of those questions here.

I was not practicing anything for maths so I thought of trying my hand on that book.

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