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Re: Is 10 the greatest common factor of x and y? [#permalink]
26 Aug 2011, 01:37

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

(1) x = 4y (2) x - 3y = 10

1. x=4y tells us x is multiple of 4 not we not sure about Y, but both divisible by 10-Not sufficient 2 x-3y=10, means x is 10 number greater than y but it can be if x 130 y can be 120 but not sure Not Sufficient 1010-1000, 210-200 not sufficient

Now combining both we get Y=10 & x=40 which means 10 is HCF

Re: Is 10 the greatest common factor of x and y? [#permalink]
26 Aug 2011, 01:55

jagdeepsingh1983 wrote:

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

(1) x = 4y (2) x - 3y = 10

1. x=4y tells us x is multiple of 4 not we not sure about Y, but both divisible by 10-Not sufficient 2 x-3y=10, means x is 10 number greater than y but it can be if x 130 y can be 120 but not sure Not Sufficient 1010-1000, 210-200 not sufficient

Now combining both we get Y=10 & x=40 which means 10 is HCF

Re: Is 10 the greatest common factor of x and y? [#permalink]
27 Aug 2011, 10:22

Berbatov wrote:

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

(1) x = 4y (2) x - 3y = 10

B is my answer.

Reason -

1. x = 4y if x = 40, y =10 -> GCF = 10 if x = 80, y =20 -> GCF = 20 ----- not sufficient.

(2) x - 3y = 10 if y = 10, y= 40 -> GCF = 10 if y = 20, y= 70 -> GCF = 10 if y = 30, y= 100 -> GCF = 10 ------- Sufficient.

Hope it helps.!

_________________

----------------------------------------------------------------------------------------- What you do TODAY is important because you're exchanging a day of your life for it! -----------------------------------------------------------------------------------------

Re: Is 10 the greatest common factor of x and y? [#permalink]
03 Nov 2011, 09:12

(1) Putting values, we can have 40,10 or 400,100. GCD can be 10 or 100. NOT SUFFICIENT. (2) x - 3y = 10, again putting values, we can have only 40,10 or 70,20. Now let us substitute x = 10a and y = 10b. 10a – 30b = 10, Thus a = (10 +30b) / 10 We can conclude for all values of b, 10 + 30b must be a multiple of 10, hence GCD of x and y is indeed 10. SUFFICIENT. Hence ans is B

Re: If both x and y are positive integers that are divisible by [#permalink]
16 Sep 2012, 04:54

Berbatov wrote:

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

(1) x = 4y (2) x - 3y = 10

If 10 is the greatest common divisor of x and y then x=10a, \, y=10b for some positive integers a and b, where a and b are co-prime (meaning their greatest common divisor is 1).

(1) Obviously not sufficient. It just states that x is 4 times bigger than y, which can hold even without the two numbers being divisible by 10. For example x=4,\,y=1.

(2) Again, not sufficient. x=3y+10, x and y not necessarily divisible by 10. For example x=13,\,y=1.

(1) and (2) together: solving the two equations we obtain unique values for x and y: x=40,\,y=10. Sufficient.

Answer C

_________________

PhD in Applied Mathematics Love GMAT Quant questions and running.

Re: If both x and y are positive integers that are divisible by [#permalink]
16 Sep 2012, 06:01

Guess they have mentioned in the question that x & y are divisible by 10. So, x & y cant take values 13 and 1. But still I feel 2nd statement is not sufficient.

Re: If both x and y are positive integers that are divisible by [#permalink]
16 Sep 2012, 06:39

sravs27 wrote:

Guess they have mentioned in the question that x & y are divisible by 10. So, x & y cant take values 13 and 1. But still I feel 2nd statement is not sufficient.

Thanks. I am such an astronaut...

So, let's try again:

Assume x=10a and y=10b, for some positive integers a and b. x and y have 10 as their greatest common divisor if and only if a and b are co-prime (their greatest common divisor is 1).

(1) x=4y translates into 10a=40b or a=4b. a and b can be co-prime only if b=1. Not sufficient.

(2) x-3y=10 becomes 10a-30b = 10 or a-3b=1. If a and b have a common divisor d (some positive integer), then a=md and b=nd for some positive integers m and n. It follows that dm-3dn = d(m-3n)=1, which means that d must be a divisor of 1, so d=1. It means that a and b are co-prime. Sufficient.

Answer B.

_________________

PhD in Applied Mathematics Love GMAT Quant questions and running.

Re: If both x and y are positive integers that are divisible by [#permalink]
17 Sep 2012, 07:33

5

This post received KUDOS

Expert's post

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

Given: x=10m and y=10n, for some positive integers m and n.

(1) x = 4y. If x=40 and y=10, then the answer is YES but x=80 and y=20, then the answer is NO. Not sufficient.

(2) x - 3y = 10 --> 10m-3*(10n)=10 --> m-3n=1 --> m=3n+1. m and 3n are consecutive integers. Any two consecutive positive integers are co-prime, which means that they do not share any common factor but 1. For example: 3 and 4, 5 and 6, 100 and 101, are consecutive integers and they do not share any common factor bu 1. So, m and 3n do not share any common factors but 1, which means that m and n also do not share any common factors but 1, therefore the greatest common factor of x=10m and y=10n is 10. Sufficient.

Re: If both x and y are positive integers that are divisible by [#permalink]
18 Oct 2013, 23:18

Bunuel wrote:

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

Given: x=10m and y=10n, for some positive integers m and n.

(1) x = 4y. If x=40 and y=10, then the answer is YES but x=80 and y=20, then the answer is NO. Not sufficient.

(2) x - 3y = 10 --> 10m-3*(10n)=10 --> m-3n=1 --> m=3n+1.m and 3n are consecutive integers. Any two consecutive positive integers are co-prime, which means that they do not share any common factor but 1. For example: 3 and 4, 5 and 6, 100 and 101, are consecutive integers and they do not share any common factor bu 1. So, m and 3n do not share any common factors but 1, which means that m and n also do not share any common factors but 1, therefore the greatest common factor of x=10m and y=10n is 10. Sufficient.

Answer: B.

Hope it's clear.

I am Sorry I can't get it, how are m and 3n consecutive integers?

Re: If both x and y are positive integers that are divisible by [#permalink]
18 Oct 2013, 23:51

1

This post received KUDOS

suk1234 wrote:

Bunuel wrote:

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

Given: x=10m and y=10n, for some positive integers m and n.

(1) x = 4y. If x=40 and y=10, then the answer is YES but x=80 and y=20, then the answer is NO. Not sufficient.

(2) x - 3y = 10 --> 10m-3*(10n)=10 --> m-3n=1 --> m=3n+1.m and 3n are consecutive integers. Any two consecutive positive integers are co-prime, which means that they do not share any common factor but 1. For example: 3 and 4, 5 and 6, 100 and 101, are consecutive integers and they do not share any common factor bu 1. So, m and 3n do not share any common factors but 1, which means that m and n also do not share any common factors but 1, therefore the greatest common factor of x=10m and y=10n is 10. Sufficient.

Answer: B.

Hope it's clear.

I am Sorry I can't get it, how are m and 3n consecutive integers?

Thanking you in advance!

AS Bunuel Proved in the second statement m = 3n + 1 that means m is 1 greater than 3n or m and 3n are consecutive.

I hope it is clear now.

I am following GMAT quant since 2 months and I am in awe of how Bunuel goes about each question so thoroughly and perfectly. I wish I may achieve half of that.

Probably "the greatest" Mathematician I have come across all my life

Re: If both x and y are positive integers that are divisible by [#permalink]
19 Jan 2014, 07:24

Bunuel wrote:

If both x and y are positive integers that are divisible by 10, is 10 the greatest common factor of x and y?

Given: x=10m and y=10n, for some positive integers m and n.

(1) x = 4y. If x=40 and y=10, then the answer is YES but x=80 and y=20, then the answer is NO. Not sufficient.

(2) x - 3y = 10 --> 10m-3*(10n)=10 --> m-3n=1 --> m=3n+1. m and 3n are consecutive integers. Any two consecutive positive integers are co-prime, which means that they do not share any common factor but 1. For example: 3 and 4, 5 and 6, 100 and 101, are consecutive integers and they do not share any common factor bu 1. So, m and 3n do not share any common factors but 1, which means that m and n also do not share any common factors but 1, therefore the greatest common factor of x=10m and y=10n is 10. Sufficient.

Answer: B.

Hope it's clear.

We can also note on statement 2 that since X and Y are both multiples of 10 and the difference is a multiple of 10 then 10 must be their GCF, if x and y had a higher GCF say like 20,30,40 etc... then the difference of two multiples of 20 will never yield 10 as a difference, same with others

Just my 2cents

Additional note: If the second statement were x-y = 10 we could also know that the GCF cannot exceed 10 because the GCF is always less than the difference between two numbers and exactly 10 when these two numbers are consecutive integers. Therefore we would know that these two numbers x and y are consecutive integers.

But that's for another problem I guess

Hope it helps Gimme some freaking Kudos!! J

gmatclubot

Re: If both x and y are positive integers that are divisible by
[#permalink]
19 Jan 2014, 07:24