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

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If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink] New post 10 May 2010, 08:32
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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, 04:17, edited 1 time in total.
Edited the question and added the OA.
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Re: DS4 [#permalink] New post 10 May 2010, 13:38
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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.

Answer: C.
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Re: DS4 [#permalink] New post 10 May 2010, 20:01
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.

Answer: C.


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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink] New post 17 Jan 2013, 02:57
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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!

Answer: C
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink] New post 26 Feb 2013, 03:02
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] New post 26 Feb 2013, 03:07
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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] New post 02 May 2014, 18:24
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] New post 02 May 2014, 23:17
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

Hence answer is C.
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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink] New post 03 May 2014, 03:50
Expert's post
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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RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: If m > 0 and n > 0, is (m+x)/(n+x) > m/n? [#permalink] New post 03 May 2014, 05:55
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, 05:55
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