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Source: — Data Sufficiency |

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by srcc25anu » Sun Mar 24, 2013 7:25 pm
given |a-b|=|b-c|=2

a---b---c OR
a/c---b OR
c---b---a OR
b---c/a

We have to find what is |a-c| = ???

A. a < b < c
only first case satisfies this and |a-c| = 2 + 2 = 4
A is sufficient.

B. c - a < c - b => -a < -b or a > b
No information about c. also from the above cases, both case 3 and 4 satisfies this. as per case 3. |a-c| = 4 and per case 4, |a-c| = 0
hence Insufficient.

Hence Answer A.

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by aaggar7 » Sun Mar 24, 2013 7:28 pm
I think the answer is A,please correct incase I am going wrong in any step.

mod(a-b) = mod (b-c) = 2

=> a-b = b-c
=> a+c = 2b---(a)

OR

a-b = c-b

=>a-c = 0---(b)

OR

b-a = b-c
Again a=c---(c)

OR
b-a = c-b
=> a + c =2b--(d)

1.Statement 1 says that a<b<c,this means a and d are valid,and since the numbers are in evenly spaced set the diff between the two extremes will be 4 and hence is sufficient.

2.Statement 2 is not sufficient.

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by Anju@Gurome » Sun Mar 24, 2013 9:26 pm
aditya8062 wrote:If |a - b| = |b - c| = 2, what is the value of |a - c|?

(1) a < b < c
(2) c - a < c - b
Always try to think absolute value expressions in terms of the distance on the number line. That helps us to visualize the problem rather than solving algebraically.

|a - b| = |b - c| = 2
So, the distance of b from both a and c on the number line is equal to 2.
This is possible only if
  • 1. b lies between a and c on the number line
    OR
    2. a = c
In the 1st case, |a - c| = |a - b| + |b - c| = 4
In the 2nd case, |a - c| = 0

Statement 1: a < b < c
So, b lies between a and c.
--> |a - c| = 4

Sufficient

Statement 2: (c - a) < (c - b) ---> -a < -b ---> a > b
This is clearly not sufficient to determine the value of |a - c| as it is possible that either c < b < a or b < a = c

Not sufficient

The correct answer is A.
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by GMATGuruNY » Mon Mar 25, 2013 4:02 am
If |a-b| = |b-c| = 2, what is |a-c|?

1. a<b<c
2. c-a < c-b
|x-y| = the DISTANCE BETWEEN x and y.

|a-b| = |b-c| = 2, in words:
The distance between a and b is 2.
The distance between b and c is 2.

Question stem, in words:
What is the distance between a and c?

Plot the distances on a NUMBER LINE.-

Statement 1: a<b<c.
Since b is BETWEEN a and c, the number line must look like this:
a<----2---->b<----2---->c.
Thus, the distance between a and c is 4.
SUFFICIENT.

Statement 2: c-a < c-b
-a < -b
a>b.

The number line could look like this:
c<----2---->b<----2---->a.
In this case, the distance between a and c is 4.

If a=c, the number line could look like this:
b<----2---->a=c.
In this, case, the distance between a and c is 0.
INSUFFICIENT.

The correct answer is A.
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