Prime Numbers....

This topic has expert replies
Senior | Next Rank: 100 Posts
Posts: 52
Joined: Tue Jul 01, 2008 10:50 am

Prime Numbers....

by evansbd » Tue Jul 29, 2008 7:36 am
If q is a multiple of prime numbers, is q a multiple of r?

1) r < 4.

2) q = 18.


I started writing out primes and their multiples however I wanted to know if anyone could share their insight with this question.

OA to follow...

Master | Next Rank: 500 Posts
Posts: 167
Joined: Tue Feb 26, 2008 4:37 am
Thanked: 5 times
Followed by:1 members

by pranavc » Tue Jul 29, 2008 10:31 am
Is the OA C? PLease let me know.

User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

Re: Prime Numbers....

by Stuart@KaplanGMAT » Tue Jul 29, 2008 12:38 pm
evansbd wrote:If q is a multiple of prime numbers, is q a multiple of r?

1) r < 4.

2) q = 18.


I started writing out primes and their multiples however I wanted to know if anyone could share their insight with this question.

OA to follow...
Let's simplify:

"If q is a multiple of prime numbers" means "If q is an integer", since all integers are multiple of primes. (Technically, one could argue that q is a non-prime integer, but that would really be about semantics.) (Further, one could argue that this first piece of information is completely irrelevant, since we really only talk about integers when we discuss factors and multiples.)

So, the question really is:

"If q is an integer, is q a multiple of r?"

(1) r<4

Well, if q=25 and r=2, then no. If q=16 and r=2, then yes. Insufficient.

(2) q=18

No info about r, so insufficient.

Together:

Again, really depends how deeply we look at the question. Nowhere have we been told that r is a positive integer. So, if q=18 and r=1.2556121, then the answer would be "no". If q=18 and r=3, the answer would be "yes". Choose (e).

However, if we're supposed to assume (a dirty word in data sufficiency) that r is a positive integer, then r must be 1, 2 or 3, all of which give us a "yes" answer. Choose (c).

In conclusion, if this were a real GMAT question (something that's very unlikely, given the weirdness), the answer would be (e). If this is a question from a questionable source, the answer may "officially" be (c).

What's the source? Please post the source of all your questions so we can better evaluate how GMATesque the question really is.
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2621
Joined: Mon Jun 02, 2008 3:17 am
Location: Montreal
Thanked: 1090 times
Followed by:355 members
GMAT Score:780

Re: Prime Numbers....

by Ian Stewart » Tue Jul 29, 2008 3:00 pm
Stuart Kovinsky wrote: "If q is a multiple of prime numbers" means "If q is an integer", since all integers are multiple of primes.
I agree that this is a bizarre question- it's either transcribed incorrectly, or it's simply terribly written. Still, a minor point, but the above is not quite correct. Even if we only consider positive integers, 1 is not a multiple of any prime. So if we know q is a positive integer (not something we're told, to be sure, but it's information you'd always find in a GMAT divisibility question), and we know 'q is a multiple of prime numbers', that means that q is an integer greater than 1. That doesn't affect the solution at all, however, and I can guarantee that you will never see a statement like 'q is a multiple of prime numbers' on the GMAT.
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

Senior | Next Rank: 100 Posts
Posts: 52
Joined: Tue Jul 01, 2008 10:50 am

by evansbd » Thu Jul 31, 2008 7:58 am
https://www.4tests.com/exams/questions.asp?googlebot=31

Question # 16

This site gives a practice test, a very simple one, with some strangely worded questions sprinkled in like the one above.

The official answer is E.

I picked C like alot of posters.

User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

by Stuart@KaplanGMAT » Thu Jul 31, 2008 5:48 pm
evansbd wrote:https://www.4tests.com/exams/questions.asp?googlebot=31

Question # 16

This site gives a practice test, a very simple one, with some strangely worded questions sprinkled in like the one above.

The official answer is E.

I picked C like alot of posters.
Again, it's a crappy question, I wouldn't have much faith in anything from that source (but at least they got the answer right).
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course