If a and b are positive integers divisible by 6, is 6 the greatest common divisor of a and b?
(1) a = 2b + 6
(2) a = 3b
OA is A
Official Explanation for this -
[spoiler]MGMAT says that for a and b to always have 6 as the GCF, they have to be mutually prime.
Hence, Statement 1 is sufficient to answer. This can be tested by plugging in numbers also.
My question is, is it right for to assume that any question which asks to check if a number is the GCF of two numbers can be solved using the mutually prime property?
Or is this dependent on the statements given? in this case statement A. Also, please do share other useful divisibility and primes properties that can be applied for DS questions.[/spoiler]
Divisibility & Primes - GCF (Tricky)
This topic has expert replies
-
- Senior | Next Rank: 100 Posts
- Posts: 69
- Joined: Sat Jul 31, 2010 9:27 pm
- Thanked: 2 times
- Followed by:4 members
- GMAT Score:640
- phanideepak
- Senior | Next Rank: 100 Posts
- Posts: 77
- Joined: Wed Apr 27, 2011 6:13 am
- Location: Hyderabad
- Thanked: 10 times
- Followed by:2 members
- GMAT Score:730
We know that both a and b are divisible by 6 so lets assume that a = 6x and b = 6y
Now all we have to do is prove that x and y have no common factors except 1
1 : a = 2b +6
6x = 12y +6
so x = 2y + 1
Intuitively u can see that for ANY value of y, x & Y can never have the same factors except 1. You can check by substitution too
for example y = 1 so x = 3 no common factors
y = 2 x = 5 again no common factors
y = 7 x = 15 again no common factors
So A is sufficient
B : a = 3b
lets take a,b = 6,18
in this case the GCF is 6
But if we take a,b = 12,36
here the GCF is 12
So B alone is not sufficient.
so IMO the answer is A
Now all we have to do is prove that x and y have no common factors except 1
1 : a = 2b +6
6x = 12y +6
so x = 2y + 1
Intuitively u can see that for ANY value of y, x & Y can never have the same factors except 1. You can check by substitution too
for example y = 1 so x = 3 no common factors
y = 2 x = 5 again no common factors
y = 7 x = 15 again no common factors
So A is sufficient
B : a = 3b
lets take a,b = 6,18
in this case the GCF is 6
But if we take a,b = 12,36
here the GCF is 12
So B alone is not sufficient.
so IMO the answer is A
- manpsingh87
- Master | Next Rank: 500 Posts
- Posts: 436
- Joined: Tue Feb 08, 2011 3:07 am
- Thanked: 72 times
- Followed by:6 members
a=6k; b=6m;Jayanth2689 wrote:If a and b are positive integers divisible by 6, is 6 the greatest common divisor of a and b?
(1) a = 2b + 6
(2) a = 3b
OA is A
Official Explanation for this -
[spoiler]MGMAT says that for a and b to always have 6 as the GCF, they have to be mutually prime.
Hence, Statement 1 is sufficient to answer. This can be tested by plugging in numbers also.
My question is, is it right for to assume that any question which asks to check if a number is the GCF of two numbers can be solved using the mutually prime property?
Or is this dependent on the statements given? in this case statement A. Also, please do share other useful divisibility and primes properties that can be applied for DS questions.[/spoiler]
1) a=2b+6;
b=6m;
a=12m+6;
=6(2m+1);
now for different values of m , 6m and 6(2m+1) will have 6 as a GCD, hence 1 alone is sufficient to answer the question.
2)
a=3b;
=3*6m;
=18m;
b=6m,
now for m=1 GCD is 6, and for m=2 GCD is 12 hence 2 alone is not sufficient to answer the question..
therefore answer should be A
O Excellence... my search for you is on... you can be far.. but not beyond my reach!
- phanideepak
- Senior | Next Rank: 100 Posts
- Posts: 77
- Joined: Wed Apr 27, 2011 6:13 am
- Location: Hyderabad
- Thanked: 10 times
- Followed by:2 members
- GMAT Score:730
Jayanth may be by meaning mutually prime numbers he means that the numbers are co-primes
Co-primes are the numbers which have no common factors except 1.
here we know that a = 6x and b= 6y so if we prove that x and y are co-primes we have the solution and I have done the same in my solution above. I forgot the term co-primes and thank you for helping me remember that
Co-primes are the numbers which have no common factors except 1.
here we know that a = 6x and b= 6y so if we prove that x and y are co-primes we have the solution and I have done the same in my solution above. I forgot the term co-primes and thank you for helping me remember that
-
- Senior | Next Rank: 100 Posts
- Posts: 69
- Joined: Sat Jul 31, 2010 9:27 pm
- Thanked: 2 times
- Followed by:4 members
- GMAT Score:640
yes phanideepak, i was intending the same!phanideepak wrote:Jayanth may be by meaning mutually prime numbers he means that the numbers are co-primes
Co-primes are the numbers which have no common factors except 1.
here we know that a = 6x and b= 6y so if we prove that x and y are co-primes we have the solution and I have done the same in my solution above. I forgot the term co-primes and thank you for helping me remember that
- cans
- Legendary Member
- Posts: 1309
- Joined: Mon Apr 04, 2011 5:34 am
- Location: India
- Thanked: 310 times
- Followed by:123 members
- GMAT Score:750
b)a=3bIf a and b are positive integers divisible by 6, is 6 the greatest common divisor of a and b?
(1) a = 2b + 6
(2) a = 3b
b is the GCD. if b=6, GCD=6 and if b=12;GCD=12
Insufficient
a)a=2b+6
b=6k (where k is an integer)
a=6(2k+1)
k and (2k+1) have G.C.D of 1
Thus a,b GCD = 6
IMO A
If my post helped you- let me know by pushing the thanks button
Contact me about long distance tutoring!
[email protected]
Cans!!
Contact me about long distance tutoring!
[email protected]
Cans!!
-
- Senior | Next Rank: 100 Posts
- Posts: 69
- Joined: Sat Jul 31, 2010 9:27 pm
- Thanked: 2 times
- Followed by:4 members
- GMAT Score:640
ah..i did not look at this way!cans wrote:b)a=3bIf a and b are positive integers divisible by 6, is 6 the greatest common divisor of a and b?
(1) a = 2b + 6
(2) a = 3b
b is the GCD. if b=6, GCD=6 and if b=12;GCD=12
Insufficient
a)a=2b+6
b=6k (where k is an integer)
a=6(2k+1)
k and (2k+1) have G.C.D of 1
Thus a,b GCD = 6
IMO A
-
- Senior | Next Rank: 100 Posts
- Posts: 69
- Joined: Sat Jul 31, 2010 9:27 pm
- Thanked: 2 times
- Followed by:4 members
- GMAT Score:640
Hi experts! just had a quick question regarding the problem in this thread (posted by me 6 months back)
If 6 IS the GCF of a and b, how can a and b be mutually prime?
Don't mutually prime numbers have just 1 as a common factor? In this case they also have 2 and 3 as common factors too!
Please do correct me if i'm wrong!
If 6 IS the GCF of a and b, how can a and b be mutually prime?
Don't mutually prime numbers have just 1 as a common factor? In this case they also have 2 and 3 as common factors too!
Please do correct me if i'm wrong!
GMAT/MBA Expert
- Mike@Magoosh
- GMAT Instructor
- Posts: 768
- Joined: Wed Dec 28, 2011 4:18 pm
- Location: Berkeley, CA
- Thanked: 387 times
- Followed by:140 members
This a reply to Jayanth2689's question
First of all, I want to say: great work in this thread! I agree with the answer of A that all participants have gotten and have explained very well.
In the original post, you said:
MGMAT says that for a and b to always have 6 as the GCF, they have to be mutually prime.
I think you might have been misreading/misquoting something from the MGMAT solution there.
If a and b have a GCF of 6, then a and b are most definitely not mutually prime.
BUT, if a and b have a GCF of 6, and if 6 the greatest common divisor of a and b, then that means if we write a = 6x and b = 6y, then we know that x and y must be mutually prime.
Mutually prime does play an important role in the problem, but it applies to x & y, not to a & b.
Does that make sense?
Mike
First of all, I want to say: great work in this thread! I agree with the answer of A that all participants have gotten and have explained very well.
In the original post, you said:
MGMAT says that for a and b to always have 6 as the GCF, they have to be mutually prime.
I think you might have been misreading/misquoting something from the MGMAT solution there.
If a and b have a GCF of 6, then a and b are most definitely not mutually prime.
BUT, if a and b have a GCF of 6, and if 6 the greatest common divisor of a and b, then that means if we write a = 6x and b = 6y, then we know that x and y must be mutually prime.
Mutually prime does play an important role in the problem, but it applies to x & y, not to a & b.
Does that make sense?
Mike
Magoosh GMAT Instructor
https://gmat.magoosh.com/
https://gmat.magoosh.com/
- [email protected]
- Legendary Member
- Posts: 934
- Joined: Tue Nov 09, 2010 5:16 am
- Location: AAMCHI MUMBAI LOCAL
- Thanked: 63 times
- Followed by:14 members
If a and b are positive integers divisible by 6, is 6 the greatest common divisor of a and b?
(1) a = 2b + 6
(2) a = 3b
OA is A
Basically the first statement says a clear no for whatever the numbers are as they are a set of co-primes...
Statement 2, the value changes from something to something...
(1) a = 2b + 6
(2) a = 3b
OA is A
Basically the first statement says a clear no for whatever the numbers are as they are a set of co-primes...
Statement 2, the value changes from something to something...
IT IS TIME TO BEAT THE GMAT
LEARNING, APPLICATION AND TIMING IS THE FACT OF GMAT AND LIFE AS WELL... KEEP PLAYING!!!
Whenever you feel that my post really helped you to learn something new, please press on the 'THANK' button.
LEARNING, APPLICATION AND TIMING IS THE FACT OF GMAT AND LIFE AS WELL... KEEP PLAYING!!!
Whenever you feel that my post really helped you to learn something new, please press on the 'THANK' button.
GMAT/MBA Expert
- lunarpower
- GMAT Instructor
- Posts: 3380
- Joined: Mon Mar 03, 2008 1:20 am
- Thanked: 2256 times
- Followed by:1535 members
- GMAT Score:800
i received a private message regarding this question.
mike --
mike --
this is correct -- and it's also what's in the MGMAT answer key. therefore, if the original poster somehow inferred that a and b themselves are relatively prime, then the original poster must have misread the explanation.Mike@Magoosh wrote:BUT, if a and b have a GCF of 6, and if 6 the greatest common divisor of a and b, then that means if we write a = 6x and b = 6y, then we know that x and y must be mutually prime.
Mutually prime does play an important role in the problem, but it applies to x & y, not to a & b.
Does that make sense?
Mike :)
Ron has been teaching various standardized tests for 20 years.
--
Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
--
Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
--
Learn more about ron
--
Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
--
Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
--
Learn more about ron
-
- Senior | Next Rank: 100 Posts
- Posts: 69
- Joined: Sat Jul 31, 2010 9:27 pm
- Thanked: 2 times
- Followed by:4 members
- GMAT Score:640
Thanks Mike! Yes it does!!Mike@Magoosh wrote:This a reply to Jayanth2689's question
First of all, I want to say: great work in this thread! I agree with the answer of A that all participants have gotten and have explained very well.
In the original post, you said:
MGMAT says that for a and b to always have 6 as the GCF, they have to be mutually prime.
I think you might have been misreading/misquoting something from the MGMAT solution there.
If a and b have a GCF of 6, then a and b are most definitely not mutually prime.
BUT, if a and b have a GCF of 6, and if 6 the greatest common divisor of a and b, then that means if we write a = 6x and b = 6y, then we know that x and y must be mutually prime.
Mutually prime does play an important role in the problem, but it applies to x & y, not to a & b.
Does that make sense?
Mike
-
- Senior | Next Rank: 100 Posts
- Posts: 69
- Joined: Sat Jul 31, 2010 9:27 pm
- Thanked: 2 times
- Followed by:4 members
- GMAT Score:640
Thank you Ron! Yes it seems i had misunderstood/misread the MGMAT explanation! my bad! thanks again!lunarpower wrote:i received a private message regarding this question.
mike --this is correct -- and it's also what's in the MGMAT answer key. therefore, if the original poster somehow inferred that a and b themselves are relatively prime, then the original poster must have misread the explanation.Mike@Magoosh wrote:BUT, if a and b have a GCF of 6, and if 6 the greatest common divisor of a and b, then that means if we write a = 6x and b = 6y, then we know that x and y must be mutually prime.
Mutually prime does play an important role in the problem, but it applies to x & y, not to a & b.
Does that make sense?
Mike
GMAT/MBA Expert
- lunarpower
- GMAT Instructor
- Posts: 3380
- Joined: Mon Mar 03, 2008 1:20 am
- Thanked: 2256 times
- Followed by:1535 members
- GMAT Score:800
you're welcome.
Ron has been teaching various standardized tests for 20 years.
--
Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
--
Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
--
Learn more about ron
--
Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
--
Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
--
Learn more about ron