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# x and y are positive integers. If the greatest common divisor of x and

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CEO
Joined: 12 Sep 2015
Posts: 2988
Location: Canada
x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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03 Feb 2017, 07:36
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54% (02:28) correct 46% (02:30) wrong based on 179 sessions

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x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

*Kudos for all correct solutions

_________________

Brent Hanneson – GMATPrepNow.com

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Director
Joined: 05 Mar 2015
Posts: 990
Re: x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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03 Feb 2017, 07:56
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3
GMATPrepNow wrote:
x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

*Kudos for all correct solutions

GCD of x & 3y= 9
LCM of 3x & 9y =81
thus LCM of x & 3y= 27
Number = LCM * GCD
x*3y= 27*9 = 3^5
xy= 3^4

so 81xy = 3^4*3^4 = 3^8

Ans D
##### General Discussion
Manager
Joined: 17 May 2015
Posts: 237
Re: x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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03 Feb 2017, 07:55
2
1
GMATPrepNow wrote:
x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

*Kudos for all correct solutions

GCD of x and 3y is 9.

Let x = 9a and 3y = 9b or y = 3b, where a and b are co-prime numbers.

LCM of 3x and 9y = 81(given) LCM(3x,9y) = LCM(3*9a, 9*3b) = 27ab = 81 ==> ab =3

81xy = 81*9a*3b = 81*9*3*3 = $$3^{8}$$
CEO
Joined: 12 Sep 2015
Posts: 2988
Location: Canada
Re: x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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03 Feb 2017, 11:47
1
Top Contributor
3
GMATPrepNow wrote:
x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

If the greatest common divisor (GCD) of x and 3y is 9, then the GCD of 3x and 9y is 27
Given: the least common multiple (LCM) of 3x and 9y is 81

Nice rule: (GCD of j and k)(LCM of j and k) = jk
So, (27)(81) = (3x)(9y)
Rewrite as: (3^3)(3^4) = 27xy

Our goal is to determine the value of 81xy.
So, take (3^3)(3^4) = 27xy and multiply both sides by 3 to get: (3)(3^3)(3^4) = (3)(27xy)

Simplify: 3^8 = 81xy

Answer: D

Cheers,
Brent
_________________

Brent Hanneson – GMATPrepNow.com

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CEO
Joined: 12 Sep 2015
Posts: 2988
Location: Canada
x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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03 Feb 2017, 12:00
Top Contributor
GMATPrepNow wrote:
x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

When it comes to solving integer properties questions, it's often useful to be able to come up with values that meet the given conditions.
For example, if we're told that positive integers j and k have a greatest common divisor of 15, what are some possible values of j and k? Can you quickly come up with 3 or 4 pairs of values?
Some possibilities are: j = 15 and k = 15 (easy!), or j = 30 and k = 15, or j = 30 and k = 45, or j = 15 and k = 150, etc.

What about the given question? Can you find values of x and y such that the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81?
How about x = 27 and y = 3?
Or perhaps x = 9 and y = 9?

Once we're able to identify values that satisfy the given information, it's easy to determine the value of 81xy

If we use x = 27 and y = 3, then 81xy = (81)(27)(3) = (3^4)(3^3)(3^1) = 3^8
If we use x = 9 and y = 9, then 81xy = (81)(9)(9) = (3^4)(3^2)(3^2) = 3^8

Cheers,
Brent
_________________

Brent Hanneson – GMATPrepNow.com

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VP
Joined: 07 Dec 2014
Posts: 1102
x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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03 Feb 2017, 19:04
1
1
GMATPrepNow wrote:
x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

*Kudos for all correct solutions

if gcd of x and 3y=9,
and lcm of 3x and 9y=81,
and problem does not require unique values for x and y,
then value for both x and y can be 9
81xy=81*9^2=6561=3^8
D
Intern
Joined: 24 Jan 2018
Posts: 15
Re: x and y are positive integers. If the greatest common divisor of x and  [#permalink]

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23 Mar 2018, 19:34
1
GMATPrepNow wrote:
x and y are positive integers. If the greatest common divisor of x and 3y is 9, and the least common multiple of 3x and 9y is 81, then what is the value of 81xy?

A) $$3^5$$

B) $$3^6$$

C) $$3^7$$

D) $$3^8$$

E) $$3^9$$

*Kudos for all correct solutions

I figured that X could be 9 given the greatest common divisor of x and 3y is 9, and it follows that if least common multiple of 3x (27) and 9y is 81 then y could be 9, from there 81xy = 3^4 * 3^2 * 3^2 = 3^8 Answer choice D
Re: x and y are positive integers. If the greatest common divisor of x and &nbs [#permalink] 23 Mar 2018, 19:34
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# x and y are positive integers. If the greatest common divisor of x and

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