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Manager  Joined: 10 Oct 2005
Posts: 103
Location: Hollywood
If M is the least common multiple of 90,196, and 300, which  [#permalink]

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If M is the least common multiple of 90,196, and 300, which of the following is NOT a factor of M?

A. 600
B. 700
C. 900
D. 2,100
E. 4,900

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Originally posted by TOUGH GUY on 27 Nov 2005, 07:43.
Last edited by Bunuel on 07 Feb 2014, 10:21, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
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IF M is the least common multiple of 90,196 and 300, which of the following is NOT a factor of M?

A- 600
B- 700
C- 900
D- 2100
E- 4900

90 = 2 * 3 * 3 * 5
196 = 2 * 2 * 7 * 7
300 = 2 * 2 * 3 * 5 * 5
-----------------------------------------
LCM = 2 * 2 * 3 * 3 * 5 * 5 * 7 * 7
(TWO 2, TWO 3, TWO 5, TWO 7)

600 = 2 * 2 * 2 * 3 * 5 * 5
700 = 2 * 2 * 5 * 5 * 7
900 = 2 * 2 * 3 * 3 * 5 * 5
2100 = 2 * 2 * 3 * 5 * 5 * 7
4900 = 2 * 2 * 5 * 5 * 7 * 7
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General Discussion
Director  Joined: 24 Oct 2005
Posts: 508
Location: London

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Yes, got the same ans.
M = 2*2*3*3*5*5*7*7

600 = 2*2*2*5*5*3 -- one 2 is left out here. So that is the answer.
Director  Joined: 14 Jan 2007
Posts: 600

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first calculate the LCM of the given numbers

90 = 2*3*3*5
196=2*2*7*7
300=2*2*3*5*5

LCM = 2*2*3*3*5*5*7*7 this is the number M.

now check each number whether a factor of M.

Director  Joined: 30 Nov 2006
Posts: 505
Location: Kuwait

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2
2
If M is the least common multiple of 90, 196, and 300, which of the following is NOT a factor of M?

Rule of Thumb: whenever you see Least Common Multiple (LCM), think of factors and preferebly prime factorization. Every non-prime integer can be factored into only prime numbers. So lets start prime factoring:
[P.S. if you need help in prime factoring tell us]
For easy start, whenever you see an even integer, start with the prime factor 2 90: 3 x 30 --> 3 x 3 x 10 --> 3 x 3 x 2 x 5
196: 2 x 98 --> 2 x 2 x 49 --> 2 x 2 x 7 x 7
300: 2 x 150 --> 2 x 2 x 75 --> 2 x 2 x 3 x 25 --> 2 x 2 x 3 x 5 x 5

LCM of the three number must contain all prime factors of each and every one of the three numbers : 90, 196, and 300

LCM : 2 x 2 x 3 x 3 x 5 x 5 x 7 x 7 = M

A factor of M must contain one or more, but limited to the available ones, of the prime factors of LCM [M]

A. 600 : 2 x 3 x 2 x 5 x 2 x 5 [ uses three 2's --> Not a Factor ]
B. 700 : 2 x 5 x 2 x 5 x 7 [ a factor of M ]
C. 900 : 3 x 3 x 2 x 2 x 5 x 5 [ a factor of M ]
D. 2,100 : 7 x 3 x 2 x 5 x 2 x 5 [ you tell me ]
E. 4,900 : 7 x 7 x 2 x 5 x 2 x 5 [ yes indeedy ]

It is A
Senior Manager  Joined: 11 Feb 2007
Posts: 287

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Thank you vshaunak and Mishari for your clear explanation!! So this is how to approach this kind of problem. I should learn more of these approaches so that I don't have to plug in numbers for number property probs!
Manager  Joined: 30 Sep 2008
Posts: 89
Re: If M is the least common multiple of 90, 196, and 300, which  [#permalink]

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90 = 3^2 x 2 x 5
196 = 7^2 x 2^2
300 = 3 x 5^2 x 2^2

so the least common = 7^2 x 2^2 x 3^2 x 5^2 = 7^2 x 3^2 x 100 = (7x3)^2 x 100

A. 6 = 3 x 2 <--not
B. 7 <-ok
C. 9 = 3^2 <- ok
D. 21 = 7 x 3 <- ok
E. 49 = 7^2 <-ok

the answer is A
VP  Joined: 17 Jun 2008
Posts: 1163
Re: If M is the least common multiple of 90, 196, and 300, which  [#permalink]

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lylya4 wrote:
90 = 3^2 x 2 x 5
196 = 7^2 x 2^2
300 = 3 x 5^2 x 2^2

so the least common = 7^2 x 2^2 x 3^2 x 5^2 = 7^2 x 3^2 x 100 = (7x3)^2 x 100

A. 6 = 3 x 2 <--not
B. 7 <-ok
C. 9 = 3^2 <- ok
D. 21 = 7 x 3 <- ok
E. 49 = 7^2 <-ok

the answer is A

I think, you missed one 2 in the least common multiple. It will be 7^2 * 5^2 * 3^2 * 2^3.

To me, all the answer choices are factors of this least common multiple. Is there any typo in the question?
Intern  Joined: 29 Sep 2008
Posts: 36
Re: If M is the least common multiple of 90, 196, and 300, which  [#permalink]

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scthakur wrote:
lylya4 wrote:
90 = 3^2 x 2 x 5
196 = 7^2 x 2^2
300 = 3 x 5^2 x 2^2

so the least common = 7^2 x 2^2 x 3^2 x 5^2 = 7^2 x 3^2 x 100 = (7x3)^2 x 100

A. 6 = 3 x 2 <--not
B. 7 <-ok
C. 9 = 3^2 <- ok
D. 21 = 7 x 3 <- ok
E. 49 = 7^2 <-ok

the answer is A

I think, you missed one 2 in the least common multiple. It will be 7^2 * 5^2 * 3^2 * 2^3.

To me, all the answer choices are factors of this least common multiple. Is there any typo in the question?

A.

LCM contains 2 2's whereas 600 = 25 * 3 * 8. thus cannot be a factor.
Intern  Joined: 04 May 2011
Posts: 8
Re: A concept math, pls help  [#permalink]

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1
The key to this problem is breaking down the numbers into its prime factors. After that its a piece of cake!

90 -> 3*3*5*2
196 -> 2*2*7*7
300 -> 3*2*2*5*5

M(LCM) = multiply all the factors (pick the highest power of the common factor)

M(LCM) = $$2^2*3^2*5^2*7^2$$

The question asks which is NOT A FACTOR of M:
Only A(600) which has an additional factor 2 is not a factor of M
600 -> $$2^3*3*5^2$$ => it has an additional factor of 2 which is not present in M(only 2 factors of '2' is present here)
Intern  Joined: 09 Feb 2011
Posts: 8
Re: A concept math, pls help  [#permalink]

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Just because I've into solving multiple problems today.

Here's how I think of it: you essentially need to pull out the primes one by one from 900, 196 and 300 respectively.
The easiest way for me to do this is pull out tens first (which consists of 2*5, both primes)
so
900 - 2 * 5 * 2 * 5 , leaving a 9 which is 3 *3 so you have 2, 2, 3, 3, 5, 5
300 - 2 *5 * 2 * 5, leaving 3 so 2, 2, 3, 5, 5
196 is trickier. Up until today I didn't know it was the square of 14 but once you know that:
196 = 2 * 7 * 2 * 7 so 2, 2, 7, 7
then make a list like srivats212 said but including the the number as much as it appears in any given number
so 2, 2, 3, 3, 5, 5, 7, 7

Now find the number that requires any prime number to pop up more than it does in the bolded list.

600 = 2 * 5 * 2 * 5 * 2 * 3, so it needs 3 2's and you only have 2 twos in the bolded list, so isn't a factor of M.

I think if you want to do these things fast it's best to memorize all the squares up to 20. Which I haven't bothered up until now either, just did this in excel.
11 121
12 144
13 169
14 196
15 225
16 256
17 289
18 324
19 361
20 400
These numbers alone or multiples of these numbers seem to pop up a lot I suppose in other types of problems too. But for multiple/factor problems all you need to do is figure out the prime factors of the original number. For 324, the square root is 18. Then prime factors of 324 are 2 * 9 * 2 * 9. But memorization of the squares is essential for pattern recognition (and hence completion of problems in 2 minutes or less) I'm starting to realize.
Manager  Joined: 04 Apr 2010
Posts: 117
Re: A concept math, pls help  [#permalink]

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A should the answer. We can get if by combined factorization. 600 is the only # that we can't get from combined factorization to calculate LCM.
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Intern  Joined: 01 Aug 2006
Posts: 31
Re: IF M is the least common multiple of 90,196 and 300, which  [#permalink]

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90 = 2 * 3^2 * 5
196 = 2^2 * 7^2
300 = 2^2 * 3 * 5^2
LCM contains the HIGHEST power of EVERY number present in prime factorization.
M = LCM = 2^2 * 3^2 *5^2 * 7^2.

Look @ answer options. A: 600 = 2^3 * 3 * 5^2 ->Cannot be a factor because of 2^3.
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Re: If M is the least common multiple of 90,196, and 300, which  [#permalink]

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90 = 2 * 3 * 3 * 5

196 = 2 * 2 * 7 * 7

300 = 2 * 2 * 3 * 5 * 5

Only 2 is common in all the 3 factorization

So, LCM =$$\frac{90 * 196 * 300}{8} = 45 * 49 * 300$$(No need of further calculation)

Testing Option I > 600. It is NOT the factor

Answer = A = 600
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Re: If M is the least common multiple of 90, 196, and 300, which  [#permalink]

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1
Here is @lyla4's answer in a manner understandable to late bloomers like me LCM is the product of the greatest power of each prime that appears in any of the numbers.
90 = 3^2 x 2 x 5
196 = 7^2 x 2^2
300 = 3 x 5^2 x 2^2

So the least common multiple is = 7^2 x 2^2 x 3^2 x 5^2 = (7x3)^2 x 100
Divide each answer choice and the LCM by 100
Therefore LCM = (7x3)^2

Now we are looking for the choice in which, LCM/choice is not an integer.

A. 6 = 3 x 2 <--not
B. 7 <-ok
C. 9 = 3^2 <- ok
D. 21 = 7 x 3 <- ok
E. 49 = 7^2 <-ok

the answer is A
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Re: If M is the least common multiple of 90,196, and 300, which  [#permalink]

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TOUGH GUY wrote:
If M is the least common multiple of 90,196, and 300, which of the following is NOT a factor of M?

A. 600
B. 700
C. 900
D. 2,100
E. 4,900

The least common multiple of 90, 196, and 300 = (2)(2)(3)(3)(5)(5)(7)(7)
So, M = (2)(2)(3)(3)(5)(5)(7)(7)

Now check the answer choices...
A) 600
600 = (2)(2)(2)(3)(5)(5)
For 600 to be a factor of M, there must be three 2's, one 3 and two 5's "hiding" in the prime factorization of M. Since, M only has two 2's in its prime factorization, 600 is NOT a factor of M.

For more on the relationship between factor and prime factorization, watch this video: https://www.gmatprepnow.com/module/gmat ... /video/825

ASIDE: I thought it might be useful to show one way to find the LCM of large numbers: _________________
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Re: If M is the least common multiple of 90,196, and 300, which  [#permalink]

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90 = (2^3)(3^2)(5)

196 = (2^2)(7^2)

300 = (2^2)(3)(5^2)

LCM --> (2^2)(3^2)(5^2)(7^2)

A. is the correct answer --> 600 cannot be made from the prime numbers found in the LCM shown above.
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: If M is the least common multiple of 90,196, and 300, which  [#permalink]

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Hi All,

This question is essentially about prime-factorization. Here's a simple example of that concept:

What is the least common multiple of 10 and 15. Now you probably already know that the LCM is 30, but here's WHY it's 30...

10 = (2)(5)
15 = (3)(5)

When looking for the LCM, we need to multiply all of the prime factors of the numbers involved. However, each instance of a prime that shows up in both numbers should be counted just once (here, there's one 5 in both numbers, so we count that as just ONE 5 and not two 5s). This gives us...

(2)(3)(5) = 30

We can then use those primes to figure out all of the divisors of the LCM:

1
2
3
5
(2)(3) = 6
(2)(5) = 10
(3)(5) = 15
(2)(3)(5) = 30

The exact same concept applies to this question - it's just that there's a lot more math work involved:

90 = (2)(3)(3)(5)
196 = (2)(2)(7)(7)
300 = (2)(2)(3)(5)(5)

The LCM of these three numbers will include two 2s, two 3s, two 5s and two 7s:

(2)(2)(3)(3)(5)(5)(7)(7)

At this point, you should NOT multiply those numbers together - we're just going keep them as a reference point so that we can find the one answer that is NOT a possible factor from that list:

Let's start with the easiest option first:

Answer A: 600 = (2)(2)(2)(3)(5)(5)

Notice how this number hast THREE 2s. This number is NOT possible given the list of primes that we have to work with, so it cannot be a factor of M.

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Re: If M is the least common multiple of 90,196, and 300, which  [#permalink]

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