Ok, for some reason these GCF/LCM, etc. problems get me... : DS Archive
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# Ok, for some reason these GCF/LCM, etc. problems get me...

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Manager
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Ok, for some reason these GCF/LCM, etc. problems get me... [#permalink]

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11 Apr 2007, 22:59
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Ok, for some reason these GCF/LCM, etc. problems get me... any help much appreciated. And if you have any tips I'm all ears...

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
Manager
Joined: 11 Nov 2006
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11 Apr 2007, 23:34
There is something you should know before solving this question:

LCM(x,y) * GCF(x,y) = X*Y

Coming back to the question:
(1) not sufficient to get the value of X*Y
(2) idem (1)

(1 & 2) using the above formula, you know that LCM*GCF = X*Y Sufficient, the answer is (C)

In addition, if x and y are relatively prime (e.g. consecutive numbers are relatively prime) their GCF is 1, therefore LCM(x,y) = X*Y
Director
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12 Apr 2007, 00:36
Statement1: GCF is 10. There may me numerous numbers x and y having GCF 10. So INSUFF
Statement2 : LCM is 180, there may me more than one values of x and y . So INSUFF

Taking them together:
The product of the numbers = GCF * LCM = 1800, xy = 1800
Hence SUFF

Director
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12 Apr 2007, 07:40
raconteur wrote:
Ok, for some reason these GCF/LCM, etc. problems get me... any help much appreciated. And if you have any tips I'm all ears...

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

A little late...but yeah..you've to remember this formula

LCM*GCF=product of numbers
Senior Manager
Joined: 11 Feb 2007
Posts: 352
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12 Apr 2007, 18:27
LM wrote:
raconteur wrote:
Ok, for some reason these GCF/LCM, etc. problems get me... any help much appreciated. And if you have any tips I'm all ears...

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

A little late...but yeah..you've to remember this formula

LCM*GCF=product of numbers

Cool. Nice formula. It'll save me some precious time!

One thing tho, is this LCM(x,y)*GCF(x,y) = xy ALWAYS TRUE?

if yes, then I'll be very happy in using it.
Director
Joined: 03 Sep 2006
Posts: 879
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12 Apr 2007, 21:09
ricokevin wrote:
LM wrote:
raconteur wrote:
Ok, for some reason these GCF/LCM, etc. problems get me... any help much appreciated. And if you have any tips I'm all ears...

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

A little late...but yeah..you've to remember this formula

LCM*GCF=product of numbers

Cool. Nice formula. :) It'll save me some precious time!

One thing tho, is this LCM(x,y)*GCF(x,y) = xy ALWAYS TRUE?

if yes, then I'll be very happy in using it. :-D

It is always true...you go ahead without worrying in GMAT with it.
Manager
Joined: 04 Apr 2007
Posts: 130
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13 Apr 2007, 03:31
As mentioned above, thanks for the helpful formula! Now if I run into this type of question then I'll know what to do...again, many thanks!
GMAT Club Legend
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13 Apr 2007, 05:59
Just to add in a note, to check if a formula (or one that you derived works), you can always test it out with arbitary values you pluck out of the air.

For instance, I can try x = 20, y = 24. In this case, HCF of x and y is 4 and LCM = 120. So xy must be 120*4 = 480 (check 20*24 = 480)! Voila !!
13 Apr 2007, 05:59
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