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Re: m = 4n + 9, where n is a positive integer. What is the greatest common [#permalink]
14 Feb 2012, 05:05

I think he means GCF.

A is sufficient because you know M is a multiple of 9. Since 9s is a multiple of 9 regardless of what s. You know N and M is consecutive multiples of 9 because their difference is 9. You know N is a multiple of 9 since if you add 9 to a number the only way it will form a multiple of 9 if it was already a multiple of 9.

Since you know their consecutive multiples of 9, their GCF is also 9.

B) Not enough info. You're not sure what number M is since, 4X+9 could be prime (13,17) or a multiple of 3 (21) or 5(25).

Re: m = 4n + 9, where n is a positive integer. What is the greatest common [#permalink]
14 Feb 2012, 05:24

5

This post received KUDOS

Expert's post

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m = 4n + 9, where n is a positive integer. What is the GCD of m and n?

(1) m = 9s, where s is a positive integer --> since m is a multiple of 9 and is equal to 4n+9, then n must also be a multiple of 9 (in order 4n+9 to be a multiple of 9). Hence m and 4n are multiples of 9 and are 9 units apart from each other, which means that the Greatest Common Divisor of m and 4n is 9, obviously the GCD of m and n will also be 9. Sufficient.

USEFUL PROPERTY: if \(a\) and \(b\) are multiples of \(k\) and are \(k\) units apart from each other then \(k\) is greatest common divisor of \(a\) and \(b\). For example if \(a\) and \(b\) are multiples of 7 and \(a=b+7\) then 7 is GCD of \(a\) and \(b\).

(2) n = 4t, where t is a positive integer. If n=4 then GCD of m and n is 1 (m=9 in this case) but if n=4*9 then GCD of m and n is 9 (m=17*9 in this case). Not sufficient.

Re: m = 4n + 9, where n is a positive integer. What is the greatest common [#permalink]
14 Feb 2012, 05:27

Expert's post

kys123 wrote:

I think he means GCF.

A is sufficient because you know M is a multiple of 9. Since 9s is a multiple of 9 regardless of what s. You know N and M is consecutive multiples of 9 because their difference is 9. You know N is a multiple of 9 since if you add 9 to a number the only way it will form a multiple of 9 if it was already a multiple of 9.

Since you know their consecutive multiples of 9, their GCF is also 9.

B) Not enough info. You're not sure what number M is since, 4X+9 could be prime (13,17) or a multiple of 3 (21) or 5(25).

Therefore answer A is sufficient.

m cannot be 17 or 21 as in this case n won't be a multiple of 4. _________________

Re: m = 4n + 9, where n is a positive integer. What is the greatest common [#permalink]
05 May 2013, 08:23

Bunuel wrote:

m = 4n + 9, where n is a positive integer. What is the GCD of m and n?

(1) m = 9s, where s is a positive integer --> since m is a multiple of 9 and is equal to 4n+9, then n must also be a multiple of 9 (in order 4n+9 to be a multiple of 9). Hence m and 4n are multiples of 9 and are 9 units apart from each other, which means that the Greatest Common Divisor of m and 4n is 9, obviously the GCD of m and n will also be 9. Sufficient.

USEFUL PROPERTY: if \(a\) and \(b\) are multiples of \(k\) and are \(k\) units apart from each other then \(k\) is greatest common divisor of \(a\) and \(b\). For example if \(a\) and \(b\) are multiples of 7 and \(a=b+7\) then 7 is GCD of \(a\) and \(b\).

(2) n = 4t, where t is a positive integer. If n=4 then GCD of m and n is 1 (m=9 in this case) but if n=4*9 then GCD of m and n is 9 (m=17*9 in this case). Not sufficient.

Answer: A.

Hi Bunnel,

Had the question stem been like this > m=3n+9, then the GCF would hav been 3 right?

Re: m = 4n + 9, where n is a positive integer. What is the greatest common [#permalink]
18 Aug 2014, 06:46

This is a problem from Manhattan advanced math.

m = 4n + 9

1) m = 9s 9s = 4n +9 n = (9s - 9) n = 9(s-1)/4 -> (s-1) must be multiple of 4 (n is integer) -> s could be 5, 9, 13, 17,... testing numbers for s, we can see that the GCD is always 9

suff

2) n = 4t testing: if t = 1, n = 4 and m = 25 -> GCD = 1 ( you could stop here if you are 100% sure about stat 1) if t = 2, n = 8 and m = 41 -> GCD = 1 if t = 3, n =12 and m = 57 -> GCD = 3

Not suff

gmatclubot

Re: m = 4n + 9, where n is a positive integer. What is the greatest common
[#permalink]
18 Aug 2014, 06:46

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