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The dark purple region on the number line above is shown in its entire

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The dark purple region on the number line above is shown in its entire [#permalink]

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The dark purple region on the number line above is shown in its entirety. This region is delineated by which of the following inequalities?

(A) 10 < |x + 10| < 80

(B) 10 < |x – 100| < 80

(C) |x – 20| < 70

(D) |x – 20| < | x – 90|

(E) |x – 55| < 35


Absolute value inequalities are a rare and challenging category of test questions. This question may look very hard, but with the proper perspective, it is relatively straightforward. For a complete discussion of this question type, as well as the OE for this question, see:
Absolute Value Inequalities

Mike :-)
[Reveal] Spoiler: OA

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Re: The dark purple region on the number line above is shown in its entire [#permalink]

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Step one: find the midpoint of the region. The midpoint, halfway between and 20 and 90, is 55. In other words, 20 and 90 have the same distance from 55, a distance of 35. These endpoints are not included, but the region includes all the points that have a distance from from x = 55 that is less than 35. Translating that into math, we get the following:
|x – 55| < 35
Answer = (E)
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Re: The dark purple region on the number line above is shown in its entire [#permalink]

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New post 10 Feb 2017, 10:31
The range we are looking for is 90>=x>=20. Both B and E offer that range. Why is B incorrect? Can an expert please advise?

Thank you
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Re: The dark purple region on the number line above is shown in its entire [#permalink]

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OreoShake wrote:
The range we are looking for is 90>=x>=20. Both B and E offer that range. Why is B incorrect? Can an expert please advise?

Thank you

Dear OreoShake,

I'm happy to respond. :-)

We are looking for an inequality that expresses exactly that range--an inequality that includes every single point on that purple line and excludes every single point not on the purpose line. (E) does this perfectly.

The problem with (B) is that it certainly includes everything on the purple line, but it also includes a bunch of other points that are not on the line. For example, the points x = 100 and x = 150 both satisfy the inequality in (B), but these are points not included in the purple region.

Under-inclusion and over-inclusion are opposites, but they are both problems. We are looking for an exact fit.

Does this make sense?
Mike :-)
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Re: The dark purple region on the number line above is shown in its entire [#permalink]

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New post 10 Feb 2017, 11:24
mikemcgarry wrote:
OreoShake wrote:
The range we are looking for is 90>=x>=20. Both B and E offer that range. Why is B incorrect? Can an expert please advise?

Thank you

Dear OreoShake,

I'm happy to respond. :-)

We are looking for an inequality that expresses exactly that range--an inequality that includes every single point on that purple line and excludes every single point not on the purpose line. (E) does this perfectly.

The problem with (B) is that it certainly includes everything on the purple line, but it also includes a bunch of other points that are not on the line. For example, the points x = 100 and x = 150 both satisfy the inequality in (B), but these are points not included in the purple region.

Under-inclusion and over-inclusion are opposites, but they are both problems. We are looking for an exact fit.

Does this make sense?
Mike :-)


Thank you Mike, I understand your explanation. Would there be any cues in gmat question stem that indicate that we need an exact range? Or are we supposed to infer that for questions that ask you to find a matching range?
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Re: The dark purple region on the number line above is shown in its entire [#permalink]

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OreoShake wrote:
Thank you Mike, I understand your explanation. Would there be any cues in gmat question stem that indicate that we need an exact range? Or are we supposed to infer that for questions that ask you to find a matching range?

Dear OreoShake,

I'm happy to respond. :-)

Mathematics is always about precision 100% of the time. As a general rule, if you are unclear how specific the question wants you to be, an one answer is an exact fit, and the other is not, the exact fit is the right answer.

I don't know if you are familiar with the verb "delineate," a somewhat difficult vocabulary word to appear in a math prompt?
This region is delineated by which of the following inequalities?
If you can't give an exact definition of this word, then that's an impediment to understanding the question.

to delineate - to show the lines around something; hence, to indicate the exact boundaries of something

If you have questions about a GMAT math problem, and you cannot give the exact definition of a word that appears in the prompt, then as part of understanding the problem, you need to go on the web and figure out the exact meaning of that word. Everything in math is precise, including the words used in a problem. You cannot afford to overlook or skip the meaning of a single syllable in a math problem.

Does all this make sense?
Mike :-)
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Re: The dark purple region on the number line above is shown in its entire [#permalink]

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Perhaps the option E could be looked at like this :
1. For +ve values of x
x-55 < 35, i.e. x<90
2. For -ve values of x
-x + 55 < 35 -->
Adjusting the -ve sign results in x - 55 > -35
i.e. x > 20
So, 20 < x <90
Re: The dark purple region on the number line above is shown in its entire   [#permalink] 20 Feb 2017, 06:20
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