"must be true" question??

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

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.