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"must be true" question??

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"must be true" question??

by ohjoon2 » Fri Jun 18, 2010 7:01 am
i'm stuck on this simple algebra inequality question..
i thought the answer was E, but my prep book says the answer is C..
but C indicates that z can be ANY number greater than -8 meaning it could be 10, or 100, or 1million.
this doesn't make any sense to me because the question stem specifically states z is less than 8.
BUT i can see WHY c might be correct because in some ways, yes z must be greater than -8
it seems that i'm thinking too inclusively..
are all GMAT Math "must be true" questions the same way?
i feel it's ambigious

If it is true that z < 8 and 2z > -4, which of the following must be true?
a) -8 < z < 4
b) z > 2
c) z > -8
d) z < 4
e) none of the above
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by mj78ind » Fri Jun 18, 2010 7:10 am
ohjoon2 wrote:i'm stuck on this simple algebra inequality question..
i thought the answer was E, but my prep book says the answer is C..
but C indicates that z can be ANY number greater than -8 meaning it could be 10, or 100, or 1million.
this doesn't make any sense to me because the question stem specifically states z is less than 8.
BUT i can see WHY c might be correct because in some ways, yes z must be greater than -8
it seems that i'm thinking too inclusively..
are all GMAT Math "must be true" questions the same way?
i feel it's ambigious

If it is true that z < 8 and 2z > -4, which of the following must be true?
a) -8 < z < 4
b) z > 2
c) z > -8
d) z < 4
e) none of the above
Dude I would change the prep book since you are better than it............
8<z<-2, none of the choices except E satisfies it and this is a pretty simple problem to goof up in by a prep book.
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by Rich@VeritasPrep » Fri Jun 18, 2010 7:10 am
Hey there,

It's not ambiguous. As you said, because -2 < z < 8, it MUST BE TRUE that -8 < z.

I think you're confusing the following two ideas, which are different:

a. -8 < z must be true
b. -8 < z gives all possible solutions

When we say it must be true that z > -8, we're not saying that z an be ANY VALUE greater than -8. We're just saying that whatever value z is, it will DEFINITELY be greater than -8.
Rich Zwelling
GMAT Instructor, Veritas Prep
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by mj78ind » Fri Jun 18, 2010 7:15 am
raz1024 wrote:Hey there,

It's not ambiguous. As you said, because -2 < z < 8, it MUST BE TRUE that -8 < z.

I think you're confusing the following two ideas, which are different:

a. -8 < z must be true
b. -8 < z gives all possible solutions

When we say it must be true that z > -8, we're not saying that z an be ANY VALUE greater than -8. We're just saying that whatever value z is, it will DEFINITELY be greater than -8.
Yup you got me there Raz!
I owe an apology to the book and ohjoon!!

Hey Raz I got you on one of the others, so we are even :)

Thanks much though seriously, your posts are good.
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by Rich@VeritasPrep » Fri Jun 18, 2010 7:20 am
You're welcome! Pleasure to help.

And yes, it's true...even teachers make mistakes.

Yes, I know...hard to believe :-P
Rich Zwelling
GMAT Instructor, Veritas Prep
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