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If m and n are positive integers, each of the following coul

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goodyear2013
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If m and n are positive integers, each of the following coul [#permalink] New post   Updated on: 26 Jan 2024, 00:52
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D
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61% (02:18) correct 39% (02:03) wrong based on 673 sessions
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If m and n are positive integers, each of the following could be equivalent to \(\sqrt{800mn}\) EXCEPT

A. \(80\sqrt{m}\)

B. \(40\sqrt{10m}\)

C. \(40m\sqrt{5}\)

D. \(20\sqrt{5m}\)

E. \(20\sqrt{2m}\)


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Hi, I want to know if this OE is the best way to solve this question, please.
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D

Originally posted by goodyear2013 on 12 Jan 2014, 14:13.
Last edited by Bunuel on 26 Jan 2024, 00:52, edited 2 times in total.
Renamed the topic and edited the question.
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PareshGmat
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   02 Jun 2014, 00:11
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Consider the options one by one:

A:

\(80\sqrt{m} = \sqrt{6400m}\)

\(\sqrt{6400m} = \sqrt{800mn}\)

If n = 8, then the condition satisfies

B:

\(40\sqrt{10m} = \sqrt{16000m}\)

\(\sqrt{16000m} = \sqrt{800mn}\)

If n = 20, then the condition satisfies

C:

\(40m\sqrt{5} = \sqrt{1600 m^2 * 5} = \sqrt{8000 m^2}\)

\(\sqrt{8000 m^2} = \sqrt{800mn}\)

If m = 1 & n = 10, then the condition satisfies

D:

\(20\sqrt{5m} = \sqrt{2000m}\)

\(\sqrt{2000m} = \sqrt{800mn}\)

2000m = 800mn

2000 = 800n

We will require to make n a fraction to satisfy the condition; m & n are +ve integers, so the condition fails

E:

\(20\sqrt{2m} = \sqrt{800m}\)

\(\sqrt{800m} = \sqrt{800mn}\)

If n = 1, then the condition satisfies

Answer = D
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   12 Jan 2014, 17:02
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I think OE solution is quickest method given the constraint that m and n need to be positive integers. Basically, whatever number is outside the square root in the answer choices is a perfect square and needs to be squared and multiplied by whatever is inside the square root....for n to be a positive integer, this number has to be divisible by 800.

Sqrt800mn = Sqrt20^2*2*m*n

solving for n

C
40m*Sqrt5 = Sqrt40^2*m^2*5 = Sqrt20^2*2^2*m^2*5
Sqrt20^2*5*2^2*m^2 = Sqrt20^2*2*m*n
n=5m (m=1, n=5...ok)

D
20sqrt5m = Sqrt5m*20^2
5m = 2*m*n
5=2n (n=(5/2)....not pos int)
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   02 Jun 2014, 00:36
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goodyear2013 wrote:
If m and n are positive integers, each of the following could be equivalent to \(\sqrt{800mn}\) EXCEPT

A. \(80\sqrt{m}\)
B. \(40\sqrt{10m}\)
C. \(40m\sqrt{5}\)
D. \(20\sqrt{5m}\)
E. \(20\sqrt{2m}\)

OE
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OE.png


Hi, I want to know if this OE is the best way to solve this question, please.


Take all the numbers inside the root

(A) 6400m which can be written as 800m(8)
(B) 16000m can be written as 800m(20)
(C) 8000m(m) can be written as 800m(10m)
(D) 2000m cannot be written as 800m x integer as 2000 is not divisible by 800
(E) 800m can be written as 800m(1)

Hence the answer is D.
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   12 Sep 2018, 11:02
m,n are positive integers

sqrt(800mn)= 20*sqrt(2*m*n)

Equate this to 20*sqrt(5*m) which gives n=5/2 which is incorrect as m,n are positive integers.
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   21 Jun 2020, 15:06
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goodyear2013 wrote:
If m and n are positive integers, each of the following could be equivalent to \(\sqrt{800mn}\) EXCEPT

A. \(80\sqrt{m}\)
B. \(40\sqrt{10m}\)
C. \(40m\sqrt{5}\)
D. \(20\sqrt{5m}\)
E. \(20\sqrt{2m}\)


\((\sqrt{800mn})^2 = 800mn\)
Test whether the SQUARE of each answer choice can be equal to \(800mn\) such that m and n are positive integers.
The correct answer will yield a NONINTEGER value for m or n.

When the question stem includes the phrase which of the following, the correct answer is often D or E.

E: \(800mn = (20\sqrt{2m})^2\)
\(800mn = 400*2m\)
\(n=1\)
Since n is a positive integer, eliminate E.

D: \(800mn = (20\sqrt{5m})^2\)
\(800mn = 400*5m\)
\(n=\frac{5}{2}\)

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D
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   07 Nov 2021, 02:13
\(\sqrt{ 800 mn } \)can be simplified to \(20\sqrt{2 m n}\)

Now the easiest way would be to square our options and our simplified question stem on both sides and check for m or n to not be integers.
i.e the equation will not hold true if either n or m is an integer.

So when we do that

In option D

on squaring 20\(\sqrt{5m}\) we get 20 x 20 x 5 x m .....(1)

and on squaring 20\(\sqrt{2 m n }\) we get 20 x 20 x 2 x m x n .....(2)

For the eqn to hold true (1) and (2) must be equal.

20 x 20 x 5 x m = 20 x 20 x 2 x m x n

we get n = 5/2 which is not an integer.

therefore option D is the correct choice

Hope this Helps
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   06 Dec 2021, 00:29
\(\sqrt{800mn}\)
10*2√2mn
20√2mn
A. \(80\sqrt{m}\)
possible when n=8

B .\(40\sqrt{10m}\)
possible when n=20

C. \(40m\sqrt{5}\)
possible when m = square value and n=10

E. \(20\sqrt{2m}\)
possible when n=1

D \(20\sqrt{5m}\)
not possible as n cannot be a fraction i.e. 5/2 as m & n are integers ; option D is correct

goodyear2013 wrote:
If m and n are positive integers, each of the following could be equivalent to \(\sqrt{800mn}\) EXCEPT

A. \(80\sqrt{m}\)
B. \(40\sqrt{10m}\)
C. \(40m\sqrt{5}\)
D. \(20\sqrt{5m}\)
E. \(20\sqrt{2m}\)

OE
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Attachment:
OE.png


Hi, I want to know if this OE is the best way to solve this question, please.
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   06 Dec 2021, 03:27
goodyear2013 wrote:
If m and n are positive integers, each of the following could be equivalent to \(\sqrt{800mn}\) EXCEPT

A. \(80\sqrt{m}\)
B. \(40\sqrt{10m}\)
C. \(40m\sqrt{5}\)
D. \(20\sqrt{5m}\)
E. \(20\sqrt{2m}\)

OE
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Attachment:
OE.png


Hi, I want to know if this OE is the best way to solve this question, please.


The best way is to take the outer number inside the root and see if it is of form 800mn.

D. 20_/5m = /400*5m = We don't see a 800 factor here. Hence, wrong.
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Re: If m and n are positive integers, each of the following coul [#permalink] New post   10 Dec 2022, 06:29
Asked: If m and n are positive integers, each of the following could be equivalent to \(\sqrt{800mn}\) EXCEPT


A. \(80\sqrt{m}\): n = 8; Possible
B. \(40\sqrt{10m}\): n=20: Possible
C. \(40m\sqrt{5}\): \(n=10\sqrt{m}\): Possible when m is a square of a positive integer
D. \(20\sqrt{5m}\): \(n=2\sqrt{5}\): Not possible since n is a positive integer
E. \(20\sqrt{2m}\) : n=1: Possible

IMO D
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Re: If m and n are positive integers, each of the following coul [#permalink]
10 Dec 2022, 06:29
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