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Re: greatest common divisor [#permalink]
19 Jan 2009, 08:38

1

This post received KUDOS

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y?

a. 5 b. 5(x-y) c. 20x d. 20y e. 35x

We are looking for a choice that CANNOT be the greatest common divisor of 35x and 20y ...which means 35x and 20y when divided by the answer choice the quotient should not be a integer. lets check

a. 5 35x/5 = 7x and 20y/5 = 4y both are integers so eliminate b. 5(x-y) when x = 2 and y = 1 it could be be the greatest common divisor ..so eliminate c. 20x when x = 1 its 20 and 20 cannot be the greatest common divisor of 35x and 20y ... or 35x/20x = 7/4 which is not a integer.

Re: greatest common divisor [#permalink]
08 Feb 2012, 02:06

4

This post received KUDOS

Expert's post

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y? A. 5 B. 5(x – y) C. 20x D. 20y E. 35x

Greatest common divisor (GCD) of 35x and 20y obviously must be a divisor of both35x and 20y, which means that \frac{35x}{GCD} and \frac{20y}{GCD} must be an integer.

If GCD=20x (option C), then \frac{35x}{20x}=\frac{7}{4}\neq{integer}, which means that 20x cannot be GCD of 35x and 20y as it is not a divisor of 35x.

Answer: C.

How about the other choices, can they be GCD of 35x and 20y?

A. 5 --> if x=y=1 --> 35x=35 and 20y=20 --> GCD(35,20)=5. Answer is YES, 5 can be GCD of 35x=35 and 20y;

B. 5(x-y) --> if x=3 and y=2 --> 35x=105 and 20y=40 --> GCD(105,40)=5=5(x-y). Answer is YES, 5(x-y) can be GCD of 35x and 20y;

D. 20y --> if x=4 and y=1 --> 35x=140 and 20y=20 --> GCD(140,20)=20=20y. Answer is YES, 20y can be GCD of 35x and 20y;

E. 35x --> if x=1 and y=7 --> 35x=35 and 20y=140 --> GCD(35,140)=35=35x. Answer is YES, 35x can be GCD of 35x and 20y.

Re: greatest common divisor [#permalink]
12 Jul 2013, 08:26

Bunuel wrote:

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y? A. 5 B. 5(x – y) C. 20x D. 20y E. 35x

Greatest common divisor (GCD) of 35x and 20y obviously must be a divisor of both35x and 20y, which means that \frac{35x}{GCD} and \frac{20y}{GCD} must be an integer.

If GCD=20x (option C), then \frac{35x}{20x}=\frac{7}{4}\neq{integer}, which means that 20x cannot be GCD of 35x and 20y as it is not a divisor of 35x.

Answer: C.

In this question for the division does it mean that both X and Y must be divisible or if any one is divisible the solution works

In option D

\frac{35X}{20Y} doesn't work according to that strategy?

Re: If x and y are positive integers, which of the following [#permalink]
12 Jul 2013, 08:32

I proceeded like this:

35x can have following prime factors : 5 ,7, x [well, x can have > 1 prime factors too; if x=6, 2 and 3 will be added to the list of prime factors]

Similarly, 20y has following prime factors : 2, 5, y [Same theory holds good for y]

the GCF has to have one 5 for sure. [IF we found any answer choices that is not a multiple of 5, it could be omitted right away]

A. 5 => We already covered that GCF has 5. Eliminate B. 5 (x -y) => If x and y were 2 and 1 respectively, this would reduce to 5. (same as answer choice A). Eliminate. C. 20x prime factors are 2, 5 and x. For 2 to be part of GCF, it must have come from x as 35 in 35x doesn't have 2. [If x had 2's then, 20x= 4 x 5 x X as GCF would not tally because, there is only two 2's in 20y] D. 20y = 2 x 2 x 5 x y ... If x were 4, this would be very possible. E. 35x = 5 * 7 * x; If y=7 and x =4, this is also possible.

There are simpler reasons already stated to say why C is the answer. But, for those who use prime factor trees to attach such problems, this is how I would explain.

Re: greatest common divisor [#permalink]
12 Jul 2013, 08:34

fozzzy wrote:

Bunuel wrote:

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y? A. 5 B. 5(x – y) C. 20x D. 20y E. 35x

Greatest common divisor (GCD) of 35x and 20y obviously must be a divisor of both35x and 20y, which means that \frac{35x}{GCD} and \frac{20y}{GCD} must be an integer.

If GCD=20x (option C), then \frac{35x}{20x}=\frac{7}{4}\neq{integer}, which means that 20x cannot be GCD of 35x and 20y as it is not a divisor of 35x.

Answer: C.

In this question for the division does it mean that both X and Y must be divisible or if any one is divisible the solution works

In option D

\frac{35X}{20Y} doesn't work according to that strategy?

hi fozzy ,

i will say that best way to undersatand the defenetions of GCF and LCM.

GCF of 2 numbers means ...biggest number which is factor of those numbers.

now hers 35x==>prime factors 5/7...and others we dont know about x now 20y==>prime factors 2/2/5..and others we dont know as we dont about y

now as lets take options C: LETS SAY 20x is GCF...THEN IT MUST BE FACTOR OF BOTH...means..==>35x/20x==>this must be integer(according to defenetion of factor)==>but when we simplify that we are getting 7/4==>fraction===>hence we are sure 100 percent that this cant be a factor of both....hence it cant be GCF.

in rest all option we unknown variables are not getting cancelled...so we are not sure in that.

hope it helps

_________________

When you want to succeed as bad as you want to breathe ...then you will be successfull....

Re: greatest common divisor [#permalink]
12 Jul 2013, 08:37

Expert's post

fozzzy wrote:

Bunuel wrote:

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y? A. 5 B. 5(x – y) C. 20x D. 20y E. 35x

Greatest common divisor (GCD) of 35x and 20y obviously must be a divisor of both35x and 20y, which means that \frac{35x}{GCD} and \frac{20y}{GCD} must be an integer.

If GCD=20x (option C), then \frac{35x}{20x}=\frac{7}{4}\neq{integer}, which means that 20x cannot be GCD of 35x and 20y as it is not a divisor of 35x.

Answer: C.

In this question for the division does it mean that both X and Y must be divisible or if any one is divisible the solution works

In option D

\frac{35X}{20Y} doesn't work according to that strategy?

Not sure I understand your question...

But notice that \frac{35x}{20y}=\frac{7x}{4y} could be an integer, for example if x=4 and y=1.

Re: greatest common divisor [#permalink]
23 Nov 2013, 01:37

Bunuel wrote:

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y? A. 5 B. 5(x – y) C. 20x D. 20y E. 35x

Greatest common divisor (GCD) of 35x and 20y obviously must be a divisor of both35x and 20y, which means that \frac{35x}{GCD} and \frac{20y}{GCD} must be an integer.

If GCD=20x (option C), then \frac{35x}{20x}=\frac{7}{4}\neq{integer}, which means that 20x cannot be GCD of 35x and 20y as it is not a divisor of 35x.

Answer: C.

How about the other choices, can they be GCD of 35x and 20y?

A. 5 --> if x=y=1 --> 35x=35 and 20y=20 --> GCD(35,20)=5. Answer is YES, 5 can be GCD of 35x=35 and 20y;

B. 5(x-y) --> if x=3 and y=2 --> 35x=105 and 20y=40 --> GCD(105,40)=5=5(x-y). Answer is YES, 5(x-y) can be GCD of 35x and 20y;

D. 20y --> if x=4 and y=1 --> 35x=140 and 20y=20 --> GCD(140,20)=20=20y. Answer is YES, 20y can be GCD of 35x and 20y;

E. 35x --> if x=1 and y=7 --> 35x=35 and 20y=140 --> GCD(35,140)=35=35x. Answer is YES, 35x can be GCD of 35x and 20y.

Hope it's clear.

Hi Bunuel, Is there a way to do this using prime factorization of 35 and 20? That's the first thing that comes to mind, but I can see how to proceed from there. Thanks,

Re: greatest common divisor [#permalink]
26 May 2014, 11:46

Bunuel wrote:

If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y? A. 5 B. 5(x – y) C. 20x D. 20y E. 35x

Greatest common divisor (GCD) of 35x and 20y obviously must be a divisor of both35x and 20y, which means that \frac{35x}{GCD} and \frac{20y}{GCD} must be an integer.

If GCD=20x (option C), then \frac{35x}{20x}=\frac{7}{4}\neq{integer}, which means that 20x cannot be GCD of 35x and 20y as it is not a divisor of 35x.

Answer: C.

How about the other choices, can they be GCD of 35x and 20y?

A. 5 --> if x=y=1 --> 35x=35 and 20y=20 --> GCD(35,20)=5. Answer is YES, 5 can be GCD of 35x=35 and 20y;

B. 5(x-y) --> if x=3 and y=2 --> 35x=105 and 20y=40 --> GCD(105,40)=5=5(x-y). Answer is YES, 5(x-y) can be GCD of 35x and 20y;

D. 20y --> if x=4 and y=1 --> 35x=140 and 20y=20 --> GCD(140,20)=20=20y. Answer is YES, 20y can be GCD of 35x and 20y;

E. 35x --> if x=1 and y=7 --> 35x=35 and 20y=140 --> GCD(35,140)=35=35x. Answer is YES, 35x can be GCD of 35x and 20y.

Hope it's clear.

Hi Bunuel,

The steps here are easy to follow but one thing that bugs me is the number selection. It's almost as if you had to KNOW the answer to select the numbers to prove the statements worth. On the GMAT, that might be a little challenging.

is there a way to do this algebraically by using Prime Boxes? Meaning, 35 has 7 and 5 as it's PF and 20 has xxx?

Re: If x and y are positive integers, which of the following [#permalink]
25 Jun 2014, 23:14

How i did this (using prime factors/prime boxes)

35x will have following prime factors (pf) : 5 ,7, x (x could be anything but we leave that for now)

20y will have following prime factors (pf) : 2, 5, y (Again y could be anything but we leave that for now)

So : GCF - 5 or 5xy

A. 5 => Eliminate as GCF can be 5

B. 5 (x -y) => Leave the option for now or pick numbers to check. I left it for later (there was no need to come back to this and check as i got C as an answer)

C. 20x = 2 * 2 * 5 * x. GCF could be 5xy but 20y already has two 2's so ideally this should have come from 35x for 2*2 to be in the GCF and hence this is the answer as this can never be the GCF

D. 20y = 2 * 2 * 5 * y ; GCF could be 5xy and if x=4 (we pick this number to prove this option incorrect), this would be true

E. 35x = 5 * 7 * x; GCF could be 5xy and if y=7 (we pick this number to prove this option incorrect), this would be true

gmatclubot

Re: If x and y are positive integers, which of the following
[#permalink]
25 Jun 2014, 23:14