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Kaplan DS p320 q24

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by itslateagain_7 » Thu Dec 09, 2010 2:24 am
Hi guys,

I've searched all over the web and was sure I could find the answer to this question but it eludes me. It's from the Kaplan math workbook, Data Sufficiency Test 1, question 24, p320.

The answer in the book is completely off, as in taken from the wrong question. This error is alluded to by Todd Scheceter in his review on Amazon (https://www.amazon.com/Kaplan-GMAT-Math- ... Descending) yet nobody has responded to his request for the answer.

To that end, I present the question in full here:

S is a set of positive integers such that if integer x is a member of S, then both x squared and x cubed are also in S. If the only member of S that is neither the square nor the cube of another member of S is called the source integer, is 8 in S?

(1) 4 is in S and is not the source integer.

(2) 64 is in S and is not the source integer.

--

Anybody care to help me by providing the solution?

--

For me, I'm having trouble figuring out what exactly the relationship between 4 and 64 are with S. I would imagine if 4 is not a source integer, then it is the square or cube of another set member; ex) the square of 2.

If 2 is in S, then its cube, 8, will be in S. So statement (1) is sufficient.

If 64 is in S, and isn't the source integer, then it is the square of another number, that being 8. 8 is in S, statement (2) is sufficient as well, and so the answer to the question is thus: "Either statement by itself is sufficient."

Does that correspond to others' thoughts?

Thank you,
Mark
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by shovan85 » Thu Dec 09, 2010 2:44 am
itslateagain_7 wrote: S is a set of positive integers such that if integer x is a member of S, then both x squared and x cubed are also in S. If the only member of S that is neither the square nor the cube of another member of S is called the source integer, is 8 in S?

(1) 4 is in S and is not the source integer.

(2) 64 is in S and is not the source integer.
(1) 4 is in S and is not the source integer.

As 4 is not source that means the source integer is SQRT(4) = 2

Thus as per rule the (2)^3 = 8 must be a part of the Set.

Thus Sufficient.

(2) 64 is in S and is not the source integer.

As 64 is not source that means the source integer can be either SQRT(64) = 8 Or CUBEROOT(64) = 4

Thus when we consider 8 as source we get definite YES to 8 is in set.

BUT when we consider 4 as source we get definite NO to 8 is in set. (as 4^2 = 16 not 8)

Thus INSUFFICIENT.

IMO A
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by diebeatsthegmat » Fri Dec 10, 2010 7:02 am
itslateagain_7 wrote:Hi guys,

I've searched all over the web and was sure I could find the answer to this question but it eludes me. It's from the Kaplan math workbook, Data Sufficiency Test 1, question 24, p320.

The answer in the book is completely off, as in taken from the wrong question. This error is alluded to by Todd Scheceter in his review on Amazon (https://www.amazon.com/Kaplan-GMAT-Math- ... Descending) yet nobody has responded to his request for the answer.

To that end, I present the question in full here:

S is a set of positive integers such that if integer x is a member of S, then both x squared and x cubed are also in S. If the only member of S that is neither the square nor the cube of another member of S is called the source integer, is 8 in S?

(1) 4 is in S and is not the source integer.

(2) 64 is in S and is not the source integer.

--

Anybody care to help me by providing the solution?

--

For me, I'm having trouble figuring out what exactly the relationship between 4 and 64 are with S. I would imagine if 4 is not a source integer, then it is the square or cube of another set member; ex) the square of 2.

If 2 is in S, then its cube, 8, will be in S. So statement (1) is sufficient.

If 64 is in S, and isn't the source integer, then it is the square of another number, that being 8. 8 is in S, statement (2) is sufficient as well, and so the answer to the question is thus: "Either statement by itself is sufficient."

Does that correspond to others' thoughts?

Thank you,
Mark
the answer is D for me
statement 1 is sufficient for sure
statement 2: 64 is in S and is not the source integer.
if 8 is the source integer so 64 is in set and 64*8 is in seth
since 64 is in set s of not source integer, is it not sufficient?
whats the answer?
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by KapTeacherEli » Fri Dec 10, 2010 9:38 am
shovan85 wrote:
itslateagain_7 wrote: S is a set of positive integers such that if integer x is a member of S, then both x squared and x cubed are also in S. If the only member of S that is neither the square nor the cube of another member of S is called the source integer, is 8 in S?

(1) 4 is in S and is not the source integer.

(2) 64 is in S and is not the source integer.
(1) 4 is in S and is not the source integer.

As 4 is not source that means the source integer is SQRT(4) = 2

Thus as per rule the (2)^3 = 8 must be a part of the Set.

Thus Sufficient.

(2) 64 is in S and is not the source integer.

As 64 is not source that means the source integer can be either SQRT(64) = 8 Or CUBEROOT(64) = 4

Thus when we consider 8 as source we get definite YES to 8 is in set.

BUT when we consider 4 as source we get definite NO to 8 is in set. (as 4^2 = 16 not 8)

Thus INSUFFICIENT.

IMO A
Hi Shovan,

Excellent explanation--you're dead on!

Mark: I hope Shovan's explanation was able to clear up any confusion, but I'm happy to answer any specific question you have. I am sorry that our explanation seems to have an error. THanks for calling it to our attention; if you send an email detailing the issue to our publishing department at [email protected], we'll make sure it's straightened out in the next printing!
Eli Meyer
Kaplan GMAT Teacher
Cambridge, MA
www.kaptest.com/gmat

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