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# If a > b and if c > d , then which of the following must be

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If a > b and if c > d , then which of the following must be  [#permalink]

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Updated on: 19 Mar 2012, 05:51
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74% (01:09) correct 26% (01:36) wrong based on 332 sessions

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If a > b and if c > d , then which of the following must be true?

A. a-b > c+d
B. a-c > b-d
C. c+d < a-b
D. b+d < a+c
E. a-c < b+d

here the answer is E but i wonder if is incorrect because we add unequal quantities but should not be a + c > b+d ???

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Originally posted by carcass on 19 Mar 2012, 05:39.
Last edited by Bunuel on 19 Mar 2012, 05:51, edited 1 time in total.
Edited the question and the OA
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19 Mar 2012, 05:49
3
4
carcass wrote:
If a > b and if c > d , then wich of the following must be true ??

1) a-b > c+d
2) a-c > b-d
3) c+d < a-b
4) b+d < a+c
5) a-c < b+d

here the answer is E but i wonder if is incorrect because we add unequal quantities but should not be a + c > b+d ???

You can only add inequalities when their signs are in the same direction:

If $$a>b$$ and $$c>d$$ (signs in same direction: $$>$$ and $$>$$) --> $$a+c>b+d$$.
Example: $$3<4$$ and $$2<5$$ --> $$3+2<4+5$$.

You can only apply subtraction when their signs are in the opposite directions:

If $$a>b$$ and $$c<d$$ (signs in opposite direction: $$>$$ and $$<$$) --> $$a-c>b-d$$ (take the sign of the inequality you subtract from).
Example: $$3<4$$ and $$5>1$$ --> $$3-5<4-1$$.

BACK TO THE ORIGINAL QUESTION:
If a > b and if c > d , then which of the following must be true?
A. a-b > c+d
B. a-c > b-d
C. c+d < a-b
D. b+d < a+c
E. a-c < b+d

Since the signs of the inequalities are in the same direction we can add them: $$a+c>b+d$$, which is the same as the option D: $$b+d < a+c$$. So, the correct answer is D, not E.

Hope its' clear.
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Re: If a > b and if c > d , then which of the following must be  [#permalink]

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19 Mar 2012, 05:58
I think answer E is wrong here.

Is a > b and if c > d , then wich of the following must be true ??

1) a-b > c+d
2) a-c > b-d
3) c+d < a-b
4) b+d < a+c
5) a-c < b+d

so i picked 100 > 1 and 3 > 2 to test E

100-3<1+2 (FALSE)

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Joined: 01 Sep 2010
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19 Mar 2012, 07:02
Bunuel wrote:
carcass wrote:
If a > b and if c > d , then wich of the following must be true ??

1) a-b > c+d
2) a-c > b-d
3) c+d < a-b
4) b+d < a+c
5) a-c < b+d

here the answer is E but i wonder if is incorrect because we add unequal quantities but should not be a + c > b+d ???

You can only add inequalities when their signs are in the same direction:

If $$a>b$$ and $$c>d$$ (signs in same direction: $$>$$ and $$>$$) --> $$a+c>b+d$$.
Example: $$3<4$$ and $$2<5$$ --> $$3+2<4+5$$.

You can only apply subtraction when their signs are in the opposite directions:

If $$a>b$$ and $$c<d$$ (signs in opposite direction: $$>$$ and $$<$$) --> $$a-c>b-d$$ (take the sign of the inequality you subtract from).
Example: $$3<4$$ and $$5>1$$ --> $$3-5<4-1$$.

BACK TO THE ORIGINAL QUESTION:
If a > b and if c > d , then which of the following must be true?
A. a-b > c+d
B. a-c > b-d
C. c+d < a-b
D. b+d < a+c
E. a-c < b+d

Since the signs of the inequalities are in the same direction we can add them: $$a+c>b+d$$, which is the same as the option D: $$b+d < a+c$$. So, the correct answer is D, not E.

Hope its' clear.

Infact Bunuel as you can see from my question above I pick the right answer D but something make me suspicious and I thought to ask here to you.

This shows how is important to rely solely and exclusively on good gmat material.

Thanks Mod.
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Re: If a > b and if c > d , then which of the following must be  [#permalink]

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31 Oct 2012, 11:47
1
Algebraic Approach
a>b ==> a-b>0 (1)
c>d ==> c-d>0 (2)
(1) + (2) ==> a-b + c-d > 0 (a+c) -(b+d)>0 ==> (b+d) < (a+c)
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If a>b and c<d, which of the following MUST be true?  [#permalink]

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31 Jul 2015, 10:54
If a>b and c<d, which of the following MUST be true?
a)a−d>c−b
b)a+d>b
c)b+c>a−d
d)b−d<a−c
e)a2+d2>b2+c2
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Re: If a > b and if c > d , then which of the following must be  [#permalink]

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31 Jul 2015, 16:01
Hi All,

To address the original prompt....In questions that include inequalities in this fashion, it's important to note that we know NOTHING about how A and B relate to C and D. For example, it could be that A is the biggest number, but it might be second biggest (behind C) or third biggest (behind C and D). As such, what MUST be true will likely involve combining the bigger variables from each pair and the smaller variables in each pair.

Since A > B and C > D, adding A and C will lead to a sum that MUST be greater than the sum of B and D.

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Re: If a > b and if c > d , then which of the following must be  [#permalink]

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03 Apr 2018, 09:30
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Re: If a > b and if c > d , then which of the following must be &nbs [#permalink] 03 Apr 2018, 09:30
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