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# Problems with LCM & GCD - need help

Author Message
VP
Joined: 13 Jun 2004
Posts: 1115
Location: London, UK
Schools: Tuck'08
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Problems with LCM & GCD - need help [#permalink]

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05 Sep 2004, 17:12
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Hi,

I'll take the GMAT the 15th of this month, I still have this week to study hard. I know most of the subjects (time/distance, average, time work, triangles, mixture, integer...) But I realize there is something which seems quite simple for everybody but that I can't do it right 100% of the time : LCM or GCD (Least common multiple and greatest common divisor)

I've already looked the lesson in gmat and in the books it's not always very clear. Can someone take 2-3 examples and show me how to find it, especially how to choose the number after you do the prime factorization...

I feel quite stupid to ask this because it seems it's very simple but I really got a problem there so I hope someone can help. thanks guys.
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5045
Location: Singapore
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05 Sep 2004, 19:27
LCM = LCM of 2 or more number is the smallest number that can be divided by those 2 or more numbers

E.g. LCM of 4,8 is 8 (not 16, not 32)

2|4,8
2|2,4
2|1,2
|1,1

So LCM = 2*2*2 = 8

8 can be divided evenly by 8
8 can be divided evenly by 4

Another e.g. LCM of 4,8,10

2|4,8,10
2|2,4,5
2|1,2,5
2|1,1,5
5|1,1,1

So LCM = 2*2*2*2*5 = 80

HCF or GCD = HCF of 2 number is the largest number that can be divided evenly into those 2 numbers

E.g. HCF of 4,8

2|4,8
2|2,4
|1,2

So HCF = 2*2 = 4

4 can be divided evenly by 4
8 can be divided evenly by 4 --> Note the difference in wording from LCM

HCF of 4,8,10

2|4,8,10
|2,4,5

So HCF = 2

4 can be divided evenly by 2
8 can be divided evenly by 2
10 can be divided evenly by 2
GMAT Club Legend
Joined: 15 Dec 2003
Posts: 4289
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05 Sep 2004, 20:17
In addition, this might be useful:

What is the value of ab?

1. a = 4m, b = 4n, and gcd(m, n) = 1
2. lcm(a, b) = 24

a*b = gcd*lcm
Basically, you would need both 1&2 to answer the question and C would be it. You can test it out with plugging in numbers if you want.
_________________

Best Regards,

Paul

VP
Joined: 13 Jun 2004
Posts: 1115
Location: London, UK
Schools: Tuck'08
Followers: 7

Kudos [?]: 45 [0], given: 0

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05 Sep 2004, 20:55
Thanks you both for your time, your help and your dedication to this forum...I'll let you guys know about my score (good or not)

This website is amazing and I think now the community is bigger and bigger, I also found a lot of other websites where some people put your link there or talk about Gmat Club...you deserve it !!!
05 Sep 2004, 20:55
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