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# The lifetimes of all batteries produced by XYZ in a year

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Manager
Joined: 22 May 2007
Posts: 213
The lifetimes of all batteries produced by XYZ in a year [#permalink]

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10 Aug 2008, 08:54
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The lifetimes of all batteries produced by XYZ in a year have a distribution that is symmetric about their mean m. If the distribution has a standard deviation of d. What percent of the distribution is greater than m+d ?

(1) 68% of the distribution lies between interval m-d and m+d inclusive
(2) 16% of the distribution is less than m-d

Senior Manager
Joined: 06 Apr 2008
Posts: 401
Re: GMATPREP DS- Can someone explain? [#permalink]

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10 Aug 2008, 09:00
aaron22197 wrote:
The lifetimes of all batteries produced by XYZ in a year have a distribution that is symmetric about their mean m. If the distribution has a standard deviation of d. What percent of the distribution is greater than m+d ?

(1) 68% of the distribution lies between interval m-d and m+d inclusive
(2) 16% of the distribution is less than m-d

C)

Distribution greater than m + d = 100 - 68 - 16 = 24
Senior Manager
Joined: 06 Apr 2008
Posts: 401
Re: GMATPREP DS- Can someone explain? [#permalink]

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10 Aug 2008, 09:21
aaron22197 wrote:
that's not the OA.

I missed the word symmetric graph. Its like a inverted parabola around mean.

If you consider symmetry then OA will be D)

However I doubt you can have distribution in GMAT.
Manager
Joined: 22 May 2007
Posts: 213
Re: GMATPREP DS- Can someone explain? [#permalink]

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10 Aug 2008, 09:45
nmohindru wrote:
aaron22197 wrote:
that's not the OA.

I missed the word symmetric graph. Its like a inverted parabola around mean.

If you consider symmetry then OA will be D)

However I doubt you can have distribution in GMAT.

This is a GMATPREP question. Can you please explain how you arrived at the solution? I know the OA. I need help understanding the question.
Director
Joined: 27 May 2008
Posts: 539
Re: GMATPREP DS- Can someone explain? [#permalink]

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10 Aug 2008, 09:53
key here is "distribution is symmetric"

statement 1 : 68% is between m-d and m+d .... greater than m+d = (100-68)/2.... suff...

statement 2 : if 16% is less than m-d .... same should be greater than m+d ... suff ....
Manager
Joined: 09 Jul 2008
Posts: 109
Location: Dallas, TX
Schools: McCombs 2011
Re: GMATPREP DS- Can someone explain? [#permalink]

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10 Aug 2008, 10:04
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aaron22197 wrote:
nmohindru wrote:
aaron22197 wrote:
that's not the OA.

I missed the word symmetric graph. Its like a inverted parabola around mean.

If you consider symmetry then OA will be D)

However I doubt you can have distribution in GMAT.

This is a GMATPREP question. Can you please explain how you arrived at the solution? I know the OA. I need help understanding the question.

I am attaching a picture taken from wikipedia to try to illustrate. Pictorial works better for me to understand.
As one can see, if the distribution is symmetric, it means that half will be above the mean and half below.

For the given proble, it means that, 32% is distributed above and below m+d and m-d respectively. i.e. 16% is greater than m+d
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Standard_deviation_illustration.gif [ 2.65 KiB | Viewed 1131 times ]

Re: GMATPREP DS- Can someone explain?   [#permalink] 10 Aug 2008, 10:04
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