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Senior Manager  Joined: 07 Aug 2011
Posts: 499
GMAT 1: 630 Q49 V27 x and y are positive integers such that x < y. If  [#permalink]

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Difficulty:   65% (hard)

Question Stats: 59% (02:27) correct 41% (02:08) wrong based on 272 sessions

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x and y are positive integers such that x < y. If $$x\sqrt{y} = 6\sqrt{6}$$, then xy could equal

A. 36
B. 48
C. 54
D. 96
E. 108

Originally posted by Lucky2783 on 15 Feb 2015, 23:24.
Last edited by Bunuel on 16 Feb 2015, 03:59, edited 1 time in total.
Renamed the topic and edited the question.
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Re: x and y are positive integers such that x < y. If  [#permalink]

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Lucky2783 wrote:
x and y are positive integers such that x < y. If x sqrt(y) = 6 sqrt(6) , then xy could equal

36
48
54
96
108

First instinct, when you see
$$x*\sqrt{y} = 6*\sqrt{6}$$, you say that x = 6, y = 6 will satisfy this. But note that x < y.

So y should be 6*Perfect square so that when you square root it, you are left with $$\sqrt{6}$$.

Try y = 6*4. Then x will be 3 so that you get 3*2 = 6 outside the square root.
x*y = 3*24 = 72 (not in the options)

Try y = 6*9. Then x will be 2 so that you get 2*3 = 6 outside the square root.
x*y = 2*54 = 108

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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: x and y are positive integers such that x < y. If  [#permalink]

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HI Lucky2783,

This question is a mix of Exponent rules and Number Properties.

We're told that X and Y are POSITIVE INTEGERS and that X < Y. We're also told that X(sqrt Y) = 6(sqrt 6).

For example:
(sqrt 50) = 5(sqrt 2)

Here, we have to go "in reverse" and put the 6 "back in" to the radical...

6(sqrt 6) = (sqrt 216)

The question asks for a possible value of XY...

We have (X)(X)(Y) = 216 as a reference

Since the answers are ALL integers, we're looking for one, that when multiplied by another positive integer, gives us 216... Notice how 108 is exactly HALF of 216....?

IF...
(X)(Y) = 108 and X=2, then (X)(X)(Y) = 216.

GMAT assassins aren't born, they're made,
Rich
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Re: x and y are positive integers such that x < y. If  [#permalink]

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X and Y are positive integer, which eliminates all negative situation.

x\sqrt{y} = 6\sqrt{6} ->
\sqrt{x*x*y} = \sqrt{6*6*6}
6*6*6 = 3*2*3*2*3*2
since x<y
-> x= 2, xy = 3*2*3*2*3 = 108
Math Expert V
Joined: 02 Aug 2009
Posts: 8285
Re: x and y are positive integers such that x < y. If  [#permalink]

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2
Lucky2783 wrote:
x and y are positive integers such that x < y. If $$x\sqrt{y} = 6\sqrt{6}$$, then xy could equal

A. 36
B. 48
C. 54
D. 96
E. 108

hi,
since it is given
$$x\sqrt{y} = 6\sqrt{6}$$...
It is clear that $$\sqrt{y} = t\sqrt{6}$$...
and t can be 2 or 3 or 6 as 6=2*3..
1)let t=3 so x will be 2.. $$\sqrt{y} = 3\sqrt{6}$$...
so y=$$(3\sqrt{6})^2$$
y=54 and xy=108.. ans E..
although we already have our ans, the two other possible values can be..
2) the other possible value is t=2 and x=3
xy=3*$$(2\sqrt{6})^2$$=3*24=72.. not a choice
3) the other possible value is t=6 and x=1
xy=1*$$(6\sqrt{6})^2$$=1*216=216.. not a choice
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Posts: 8285
x and y are positive integers such that x < y. If  [#permalink]

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EMPOWERgmatRichC wrote:
HI Lucky2783,

This question is a mix of Exponent rules and Number Properties.

We're told that X and Y are POSITIVE INTEGERS and that X < Y. We're also told that X(sqrt Y) = 6(sqrt 6).

For example:
(sqrt 50) = 5(sqrt 2)

Here, we have to go "in reverse" and put the 6 "back in" to the radical...

6(sqrt 6) = (sqrt 216)

The question asks for a possible value of XY...

We have (X)(X)(Y) = 216 as a reference

Since the answers are ALL integers, we're looking for one, that when multiplied by another positive integer, gives us 216... Notice how 108 is exactly HALF of 216....?

IF...
(X)(Y) = 108 and X=2, then (X)(X)(Y) = 216.

GMAT assassins aren't born, they're made,
Rich

hi,
the ans is correct but there are other values in choices which when multiplied by integer would give you 216..
A. 36.... 36*6=216
C. 54....54*4=216
E. 108...108*2=216...
so it is important to see which of these satisfies the equation x<y to get the correct answer...
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: x and y are positive integers such that x < y. If  [#permalink]

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Hi chetan2u,

As noted in my explanation, X and Y are both INTEGERS and X < Y; we need an answer that is in the format (X)(X)(Y) = 216

Of the 3 values that you listed, 2 of them do NOT fit that pattern.

36(6) = 216, but you would end up with (6)(6)(6) which is NOT a match (since X is NOT less than Y).

54(4) = 216, but you would end up with either (4)(4)(13.5) or (root54)(root54)(4), NEITHER of which is a match (since they both include NON-integer values).

108(2) = 216, which gives us (2)(2)(54), which IS the correct answer.

GMAT assassins aren't born, they're made,
Rich
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x and y are positive integers such that x < y. If  [#permalink]

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x is smaller than y

$$x*sqrt(y) = 6*sqrt(6) sqrt[(x^2)*y] = sqrt(36*6) sqrt[(x^2)*y] = sqrt(216)$$

ok, let's find prime factorization of 216
ok, 216 = 2 * 108
108 = 2 * 54
stop right here!
we have two 2's, and 54. since x^2 * y = 216, it might be the case that x=2 and y = 54. 2*54 = 108, and it is in the answer choices. since it is a "could be" question, there is no need to check further.
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Re: x and y are positive integers such that x < y. If  [#permalink]

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