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# Is |a - b| + |c - b|= |c -a| ?

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Math Expert
Joined: 02 Sep 2009
Posts: 51035
Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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25 May 2016, 03:51
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Difficulty:

65% (hard)

Question Stats:

58% (01:55) correct 42% (02:03) wrong based on 199 sessions

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Is |a - b| + |c - b|= |c -a| ?

(1) a < b < c
(2) ab < 0

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Math Expert
Joined: 02 Aug 2009
Posts: 7095
Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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25 May 2016, 06:11
3
1
Bunuel wrote:
Is |a - b| + |c - b|= |c -a| ?

(1) a < b < c
(2) ab < 0

An easier method would be-

$$(1) a < b < c$$
we solve MOD by CRITICAL method..
here the relative values are already given so lets work on these..

a<b means a-b will be negative so |a-b| = -(a-b)
a<c means c-a will be positive so |c-a| = c-a
b<c means c-b will be positive so |c-b| = c-b..

substitute in the main equation
$$|a - b| + |c - b|= |c -a| ............................... -(a - b) + (c - b)= (c -a).................-a+b+c-b=c-a......................c-a=c-a..............$$correct
Suff

$$(2) ab < 0$$
we do not know anything about c and even ab... a can be - and b, + or vice versa
Insuff...

A
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Joined: 25 Feb 2013
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Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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01 Jan 2018, 05:08
Bunuel wrote:
Is |a - b| + |c - b|= |c -a| ?

(1) a < b < c
(2) ab < 0

Statement 1: on a number line it can be represented as

$$a$$.........$$b$$.........$$c$$ . this implies distance between $$a$$ & $$b$$ and $$b$$ & $$c$$ is equal to distance between $$c$$ & $$a$$. Mathematically this can be written as

$$|a-b|+|c-b|=|c-a|$$. Sufficient

Statement 2: Nothing mentioned about $$c$$. Insufficient

Option A
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Joined: 15 Jan 2017
Posts: 356
Re: Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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01 Jan 2018, 06:09
niks18 wrote:
Bunuel wrote:
Is |a - b| + |c - b|= |c -a| ?

(1) a < b < c
(2) ab < 0

Statement 1: on a number line it can be represented as

$$a$$.........$$b$$.........$$c$$ . this implies distance between $$a$$ & $$b$$ and $$b$$ & $$c$$ is equal to distance between $$c$$ & $$a$$. Mathematically this can be written as

$$|a-b|+|c-b|=|c-a|$$. Sufficient

Statement 2: Nothing mentioned about $$c$$. Insufficient

Option A

hi! I have a slight issue in my understanding here. So I selected E as 1) if c>b>a we are not sure whether the exact distance is the same (it is not stated)

2) obviously not suff.

Hence both were insufficient for me.
For St 1, I'd like to know, how did you gauge that the distance is the same (also the stem asks this, does not say so).
Please let me know the mistake I am making
PS Forum Moderator
Joined: 25 Feb 2013
Posts: 1217
Location: India
GPA: 3.82
Re: Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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01 Jan 2018, 06:30
niks18 wrote:
Bunuel wrote:
Is |a - b| + |c - b|= |c -a| ?

(1) a < b < c
(2) ab < 0

Statement 1: on a number line it can be represented as

$$a$$.........$$b$$.........$$c$$ . this implies distance between $$a$$ & $$b$$ and $$b$$ & $$c$$ is equal to distance between $$c$$ & $$a$$. Mathematically this can be written as

$$|a-b|+|c-b|=|c-a|$$. Sufficient

Statement 2: Nothing mentioned about $$c$$. Insufficient

Option A

hi! I have a slight issue in my understanding here. So I selected E as 1) if c>b>a we are not sure whether the exact distance is the same (it is not stated)

2) obviously not suff.

Hence both were insufficient for me.
For St 1, I'd like to know, how did you gauge that the distance is the same (also the stem asks this, does not say so).
Please let me know the mistake I am making

why do we need exact distance here?

From the given relation it is sufficient to derive that distance between c & a is summation of distance between a & b and c & b.

For eg. if c=10 b=4 and a=1, then |c-a|=|10-9|=9; |a-b|=|1-4|=3 & |c-b|=|10-4|=6 so 3+6=9=|a-b|+|c-b|

similarly you can take any example to understand the relation.
Manager
Joined: 23 Oct 2017
Posts: 64
Re: Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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01 Jan 2018, 20:14
|a-b| + |c-b| = |c-a|
|a-b| can be viewed as distance between a & b;
|c-b| can be viewed as distance between c & b;
|c-a| can be viewed as distance between c & a;
So basically, distance between a & b + distance between c & b =distance between c & a

Indicating an arrangement: ......a.....b..........c.....
a<b<c
Manager
Joined: 04 Oct 2017
Posts: 70
Re: Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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09 Jan 2018, 06:09
Hi Bunuel,

a<b<c

Case 1
1<2<3
Substituted the values and got the values. True

Case 2
a<b<c
-3<-2<-1
Substituted and got the values.True

Hence sufficient

Option 2

a, b could be positive / negative ....Insufficient

Is this method correct???
SVP
Joined: 12 Dec 2016
Posts: 1653
Location: United States
GMAT 1: 700 Q49 V33
GPA: 3.64
Re: Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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09 Jan 2018, 07:33
Kezia9 wrote:
Hi Bunuel,

a<b<c

Case 1
1<2<3
Substituted the values and got the values. True

Case 2
a<b<c
-3<-2<-1
Substituted and got the values.True

Hence sufficient

Option 2

a, b could be positive / negative ....Insufficient

Is this method correct???

your method is counted as an education guess, but it's worth it since the problem is quite difficult.
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 6611
GMAT 1: 760 Q51 V42
GPA: 3.82
Re: Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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09 Jan 2018, 18:32
Bunuel wrote:
Is |a - b| + |c - b|= |c -a| ?

(1) a < b < c
(2) ab < 0

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.

|a-b| +|c-b| = |c-a|
⇔ |a-b|+|b-c| = | (a-b) + (b-c) |
⇔ (a-b)(b-c) ≥ 0

Condition 1)
Since a - b < 0 and b - c < 0, we have (a-b)(b-c) > 0.
The condition 1) is sufficient.

Condition 2)
Since we don't have any information about c, the condition 2) only is not sufficient.

Note. We should keep in mind the following properties.
|x+y| ≤ |x| + |y| holds always.
|x+y| = |x| + |y| ⇔ xy ≥ 0
|x+y| < |x| + |y| ⇔ xy < 0
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Manager
Joined: 29 Nov 2016
Posts: 53
Is |a - b| + |c - b|= |c -a| ?  [#permalink]

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04 Mar 2018, 00:22
Statement 1: when c>b>a then two cases
Case 1)
------0---a----b----c hence here distance of c-a will be equal to sum of distances of b-a and c-b
True
----a----b----c----0 hence distance of c-a , again will be equal to sum of distances of b-a and c-b , hence true SUFFICIENT

Statement 2:

AB<0 i.e. both A,B lie on the same side of number line
Ideally 4 cases
----C---0---B-----A here c-a is equal to sum of c-b, b-a
True
Case 2
----c--0---a---b here c-a is not equal to sum of a-b, c-b ,
False

Insufficient.

Hence Ans A.

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Is |a - b| + |c - b|= |c -a| ? &nbs [#permalink] 04 Mar 2018, 00:22
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