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GMAT 5/18
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Post subject: If x and y are positive integers, what is the value of xy?  Posted: Wed May 02, 2007 12:34 pm |
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I think this is a prime box type question, but I have no idea how to do it.
If x and y are positive integers, what is the value of xy?
(1) The greatest common factor of x and y is 10.
(2) The least common multiple of x and y is 180.
I started this question looking at statement (1). I might be wrong, but I came to the conclusion that x and y must both have 5 & 2 in their prime boxes (as this is the only way, I think, that the greatest common factor of x and y can be 10). I realised from here that I couldn't get the answer from statement (1), so that left B, C, and E.
Then, i got lost. :) Any help would be awesome. Thanks!
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Guest
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Post subject: Posted: Wed May 02, 2007 8:59 pm |
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Im not too sure but the answer could be E.
Clearly 1) and 2) each by itself is insuff
Combining,
GCF=2*5
A : B
10 10
LCM=180 = 2^2*3^2*5
Since 2*5 is already allocated, now we need to split 2*3^2
This can be done two ways
A : B
1) 3^2-9 (90) : 2 (20)
2) 2*3^2=18(180): 1 (10)
So E.
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GMAT 5/18
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Post subject: Posted: Wed May 02, 2007 10:04 pm |
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I guessed E as well, but the answer is C.
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dbernst
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Post subject: Posted: Fri May 04, 2007 12:21 am |
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| ManhattanGMAT Staff |
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Posts: 304
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I agree that, putting the statement together, the prime factors of 18 can be split two ways between the two prime boxes (which already contain 5*2):
Either you have 3*3*2 in one (x = 180) and y remains the same (y=10), or
you have 3*3 in one (x = 90) and 2 in the other (y = 20)
However, don't forget that the original question was What is the value of xy :!: In either case, the value of xy = 1800. The correct answer is C.
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Live Stronger
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Post subject: Posted: Fri Dec 26, 2008 4:28 pm |
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And also LCM(a,b) * GCD(a,b) = a*b
Stmt1 gives GCD, stmt2 gives LCM, so the answer is C
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RonPurewal
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Post subject: Posted: Sat Dec 27, 2008 7:04 pm |
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| ManhattanGMAT Staff |
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Posts: 7146
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abedinbhuiyan
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Post subject: Re: If x and y are positive integers, what is the value of xy? Posted: Fri Dec 03, 2010 9:59 am |
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| Students |
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Posts: 25
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there must be route to go to number with prime box formula..
br
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jnelson0612
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Post subject: Re: If x and y are positive integers, what is the value of xy? Posted: Mon Dec 06, 2010 7:59 am |
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| ManhattanGMAT Staff |
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Posts: 1857
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abed, please check out the link Ron has posted. We always want to focus on the quickest, most efficient way to do a problem.
_________________
Jamie Nelson
ManhattanGMAT Instructor
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capntangen
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Post subject: Re: If x and y are positive integers, what is the value of xy? Posted: Sun Jun 26, 2011 11:39 pm |
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| Students |
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Posts: 2
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What GMAT review guide would cover this topic?
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RonPurewal
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Post subject: Re: If x and y are positive integers, what is the value of xy? Posted: Wed Jun 29, 2011 6:13 am |
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| ManhattanGMAT Staff |
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Posts: 7146
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capntangen wrote: What GMAT review guide would cover this topic? number properties
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