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If m > 0 and n > 0, is (m+x)/(n+x) > m/n?

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Director
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If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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10 May 2010, 09:32
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If m > 0 and n > 0, is (m+x)/(n+x) > m/n?

(1) m < n.
(2) x > 0.
[Reveal] Spoiler: OA

Last edited by Bunuel on 30 Jul 2012, 05:17, edited 1 time in total.
Edited the question and added the OA.
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10 May 2010, 14:38
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Expert's post
If m>0 and n>0, is (m+x)/(n+x) > m/n?

(1) m < n. No info about x. Not sufficient.
(2) x >0. No info about m and n. Not sufficient.

(1)+(2) As from the above two statements nominators and denominator of both fractions are positive, we can crossmultiply --> is $$\frac{m+x}{n+x}>\frac{m}{n}$$ --> is $$(m+x)n>(n+x)m$$ --> is $$mn+xn>mn+xm$$ --> is $$x(n-m)>0$$ --> as $$x>0$$ and $$n>m$$, then $$x(n-m)>0$$ is true. Sufficient.

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10 May 2010, 21:01
Bunuel wrote:
If m>0 and n>0, is (m+x)/(n+x) > m/n?

(1) m < n. No info about x. Not sufficient.
(2) x >0. No info about m and n. Not sufficient.

(1)+(2) As from the above two statements nominators and denominator of both fractions are positive, we can crossmultiply --> is $$\frac{m+x}{n+x}>\frac{m}{n}$$ --> is $$(m+x)n>(n+x)m$$ --> is $$mn+xn>mn+xm$$ --> is $$x(n-m)>0$$ --> as $$x>0$$ and $$n>m$$, then $$x(n-m)>0$$ is true. Sufficient.

Did you score 60 in the Quant or are you working with the GMAC!!!

Awesome dexterity in giving the solutions.
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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17 Jan 2013, 03:57
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LM wrote:
If m > 0 and n > 0, is (m+x)/(n+x) > m/n?

(1) m < n.
(2) x > 0.

I used plug in...

1.
let m=3 and n=4 and x = 1
$$\frac{m}{n} = \frac{3}{4}$$ while $$\frac{m+x}{n+x}= \frac{4}{5}$$
$$\frac{3}{4} < \frac{4}{5}$$ YES!

let m=3 and n=4 and x=-1
$$\frac{m}{n} = \frac{3}{4}$$ while $$\frac{m+x}{n+x}= \frac{2}{3}$$
$$\frac{3}{4} > \frac{2}{3}$$ NO!

thus, INSUFFICIENT!

2. x > 0
From statement 1 we tested m=3 and n=4 and x=1 (see that x>0 here) and we got YES!

let m=4 and n=3
$$\frac{m}{n} = \frac{4}{3}$$ while $$\frac{m+x}{n+x}= \frac{5}{4}$$
$$\frac{4}{3} > \frac{5}{4}$$ NO!

thus, INSUFFICIENT!

Together, we combine and using statement 1 where when x>0 we get YES!

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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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26 Feb 2013, 04:02
Why can't we cross multiply in the original statement?
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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26 Feb 2013, 04:07
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Expert's post
fozzzy wrote:
Why can't we cross multiply in the original statement?

Never multiply (or reduce) an inequality by variable (or by an expression with variable) if you don't know its sign.

We don't know whether n+x is positive or negative, thus don't know whether we should flip the sign or not.
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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02 May 2014, 19:24
Hi Bunuel,

Why isn't the answer B given that in the below steps, (2) gives us the same information as in (1)?

(2)
Because we know that both m and n are positive and that x is positive, we can safely cross-multiply.
(m+x)*n > (n+x)*m
mn + xn > mn + xm
xn > xm
n > m
Because we now know that n > m, we can use the same steps that you used for C to answer the question and only (2) will be sufficient to answer the problem.
Please tell me where I am going wrong here.
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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03 May 2014, 00:17
LM wrote:
If m > 0 and n > 0, is (m+x)/(n+x) > m/n?

(1) m < n.
(2) x > 0.

Statement I is insufficient:

Let us say m = 4 and n = 5

Is (4+x)/(5+x) > 4/5?
(Take a hint from the second statement - Apply the negation test)

(4 - 4)/(5-4) is not greater than 4/5
(4 + 5)/(5+5) is greater than 4/5

Statement II is not sufficient:
(4 + 5)/(5+5) is greater than 4/5
(5 + 4)/(4 + 4) is not greater than 5/4

Combining is sufficient:
m > n and x is positive
Cross multiplying the inequality:
(mn + nx) > mn + mx
n > m which is true in statement I

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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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03 May 2014, 04:50
TooLong150 wrote:
Hi Bunuel,

Why isn't the answer B given that in the below steps, (2) gives us the same information as in (1)?

(2)
Because we know that both m and n are positive and that x is positive, we can safely cross-multiply.
(m+x)*n > (n+x)*m
mn + xn > mn + xm
xn > xm
n > m
Because we now know that n > m, we can use the same steps that you used for C to answer the question and only (2) will be sufficient to answer the problem.
Please tell me where I am going wrong here.

For (2) we don't know whether n>m.

The question asks whether (m+x)/(n+x) > m/n. For (2) when you simplify the question becomes is n>m? This is not given, that;s exactly what we need to find out.

Does this make sense?
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink]

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03 May 2014, 06:55
Yes, I realize this now, and that with (1), we know that the answer to this question statement is Yes.

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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n?   [#permalink] 03 May 2014, 06:55
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