Questions about the world of GRE Math from other sources and general math related questions.
DanC
Students
 
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Number Properties book, chapter 2 problem set, numbers 1-10

by DanC Fri Oct 24, 2014 2:36 pm

These questions all require an answer of "yes", "no" or "cannot be determined", but all the answers turn out to be either "yes" or "cannot be determined". None of the answers is "no." Are there questions like this that ever yield a "no" answer? From the answer explanations, it seems unlikely "no" would be correct for any similar question.

In question 2, for example, the fact that 80 is a factor of r but we do not know the other factors of r mean that the ? in the factor tree could be anything. That means, based on the factor foundation rule, that it is always possible (though not definite) that 15 is a factor of r because all you have to do is replace the ? with a number that has 3 as a factor (which can be done simply by using 3 to replace the ?). Then r=3X80=240, which, as the explanation tells us, proves the answer is "cannot be determined".

Seems like you can just replace the unknown factor (?) with whatever missing factor is needed (3) to make the factor in question (15) work. So when would the answer be "no"?
tommywallach
Manhattan Prep Staff
 
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Re: Number Properties book, chapter 2 problem set, numbers 1-10

by tommywallach Fri Oct 24, 2014 10:25 pm

If x is a prime number, then is 50x divisible by 15x?



Consider.



The answer here would be no, because even though x is unknown, it's the same in both cases, so the 3 in 15 can't be present in 50x.

But that's just a random example. I wouldn't overthink it. These aren't "real" GRE questions (as you know, GRE questions aren't ever asked in terms of Yes/No/CBD), so focus more on the concepts underlying the questions than on whether you'll see something similar on test day.

-t