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The greatest common factor of 16 and the positive integer n [#permalink]
21 Dec 2008, 04:18

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Difficulty:

95% (hard)

Question Stats:

49% (02:10) correct
51% (01:17) wrong based on 198 sessions

The greatest common factor of 16 and the positive integer n is 4, and the greatest common factor of n and 45 is 3. Which of the following could be the greatest common factor of n and 210?

Re: Greatest Common Factor [#permalink]
21 Dec 2008, 11:24

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I would say 42.

16 = 2^4. 16 and n have GCF as 2^2. That means, n could have only 2 twos as the factor.

45 = 5*3^2. 45 and have GCF as 3. That means, n will not have 5 as factor and only one 3 as the factor.

210 = 2*3*5*7 and since n does not have 5 as factor and n has 2^2 and 3 as factors, n could as well have 7 as a factor. Hence, 2*3*7 would be the greatest common factor.

Re: Greatest Common Factor [#permalink]
21 Dec 2008, 11:35

The greatest common factor of 16 and the positive integer n is 4, and the greatest common factor of n and 45 is 3. Which of the following could be the greatest common factor of n and 210? a) 3 b) 14 c) 30 d) 42 e) 70

Re: Greatest Common Factor [#permalink]
21 Dec 2008, 21:41

krishan wrote:

The greatest common factor of 16 and the positive integer n is 4, and the greatest common factor of n and 45 is 3. Which of the following could be the greatest common factor of n and 210? a) 3 b) 14 c) 30 d) 42 e) 70

210 = 5*2*3*7

N = 4x * 3 y = 2^2*3xy

6 must be the minimum common factor between n and 210. Answer must be divisible by 6

only 30 and 42 left. Maximum value could be 42.

If answer choice has 210 then will chose 210 _________________

Your attitude determines your altitude Smiling wins more friends than frowning

Re: Greatest Common Factor [#permalink]
22 Dec 2008, 10:53

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sidrah wrote:

im confused.

i got up to the point where 6 = lowest common factor.

then i understand that the answer has to be divisible by 6, therefore 30 or 42. why not 30?

"Which of the following could be the greatest common factor of n and 210?" Question is asking about greatest common factor.. obviously.. 42>30.. so 42 is the answer. _________________

Your attitude determines your altitude Smiling wins more friends than frowning

Re: Greatest Common Factor [#permalink]
23 Dec 2008, 11:35

krishan wrote:

The greatest common factor of 16 and the positive integer n is 4, and the greatest common factor of n and 45 is 3. Which of the following could be the greatest common factor of n and 210? a) 3 b) 14 c) 30 d) 42 e) 70

agree with 42.

16 = 2x2x2x2 45 = 3x3x5 n = 4 x 3 x k, where k is an integer 210 = 2x3x5x7

gcf of 16 and n = 4 gcf of n and 45 = 3 gcf of n and 210 = 2x3x7 (5 and 4 cannot be factors of n because 210 has only 2 as factor and n doesnot have 5 as factor) = 42

Re: Greatest Common Factor [#permalink]
01 Apr 2014, 12:35

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Re: The greatest common factor of 16 and the positive integer n [#permalink]
01 Apr 2014, 15:01

Expert's post

1

This post was BOOKMARKED

The greatest common factor of 16 and the positive integer n is 4, and the greatest common factor of n and 45 is 3. Which of the following could be the greatest common factor of n and 210?

A. 3 B. 14 C. 30 D. 42 E. 70

The greatest common factor of 2^4=16 and n is 4 --> n is a multiple of 2^2=4 but not the higher powers of 2, for example 2^3=8 or 2^4=16, because if it were then the greatest common factor of 16 and n would be more than 4;

The greatest common factor of 3^2*5=45 and n is 3 --> n is a multiple of 3 but not the higher powers of 3 and not 5, because if it were then the greatest common factor of 3^2*5=45 and n would be more than 3;

So, n is a multiple of 2^2*3=12, not a multiple of higher powers of 2 or 3, and not a multiple of 5 (so n=12x where x could be any positive integer but 2, 3, or 5). Now, as 210=2*3*5*7 then the greatest common factor of n and 210 could be 6 or 6*7=42 (if 7 is a factor of n).

Re: The greatest common factor of 16 and the positive integer n [#permalink]
19 Apr 2014, 07:26

Not a proper but a random approach:

GCD(n,16) = 4

n--------------2^4

n must at least be a multiple of 4

GCD(n,45) = 3

n----------------------5*3^2

n must have one '3' and shouldn't have 5

Applying above restrictions calculating

Max GCD(n,210)

n-------------------------2*3*5*7

N has one 3 and one 4 so

n= 3*2*2

n cannot have 5 but can have 7 so

n=2*2*3*7 --------------------2*3*5*7

Max(GCD) = 42 _________________

Rgds, TGC! _____________________________________________________________________ I Assisted You => KUDOS Please _____________________________________________________________________________

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Re: The greatest common factor of 16 and the positive integer n
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19 Apr 2014, 07:26

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