DISTRIBUTION QUESTION : GMAT Problem Solving (PS)
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# DISTRIBUTION QUESTION

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Manager
Joined: 09 Feb 2010
Posts: 70
Followers: 0

Kudos [?]: 89 [0], given: 4

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21 Jul 2010, 08:07
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Question Stats:

0% (00:00) correct 100% (00:45) wrong based on 2 sessions

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The average (arithmetic mean) of a normal distribution of a school's test scores is 65, and standard deviation of the distribution is 6.5. A student scoring a 78 on the exam is in what percentile of the school?

Choices
A 63rd percentile
B 68th percentile
C 84th percentile
D 96th percentile
E 98th percentile

can someone explain this
[Reveal] Spoiler: OA
CEO
Joined: 17 Nov 2007
Posts: 3589
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Followers: 548

Kudos [?]: 3561 [0], given: 360

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21 Jul 2010, 08:25
(78-65)/6.5 = 2 sigmas or 95.5th percentile for +-2 sigma but in our case we need only upper tail, so it's 100 - (100 - 95.5)/2 = 98th percentile

By the way, It's not a GMAT question because you don't need to remember percentiles for the normal distribution.
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CEO
Joined: 17 Nov 2007
Posts: 3589
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Followers: 548

Kudos [?]: 3561 [1] , given: 360

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21 Jul 2010, 10:28
1
KUDOS
Expert's post
You need to remember this number for 2 sigmas or use special tables. That is why it's not a GMAT question.

gurpreetsingh wrote:
how you got 95.5?

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Re: DISTRIBUTION QUESTION   [#permalink] 21 Jul 2010, 10:28
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