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The lifetimes of all the batteries produced by a certain

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The lifetimes of all the batteries produced by a certain [#permalink] New post 28 Sep 2006, 14:10
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A
B
C
D
E

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The lifetimes of all the batteries produced by a certain company in a year have a distribution that is symmetric about the mean m. If the distribution has a standard deviation of d, what percent of the distribution is greater than m +d?

1) 68 percent of the distribution lies in the interval from m-d to m+d, inclusive

2) 16 percent of the distribution is less than m-d

Please explain.
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 [#permalink] New post 28 Sep 2006, 14:30
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The lifetimes of all the batteries produced by a certain company in a year have a distribution that is symmetric about the mean m. If the distribution has a standard deviation of d, what percent of the distribution is greater than m +d?

1) 68 percent of the distribution lies in the interval from m-d to m+d, inclusive

2) 16 percent of the distribution is less than m-d

from one

32 percent of the distribution lies outside the interval m-d to m+d,

thus 32/2 = 16 percent is greater than m+d...suff

from two

if 16 percent of the distribution is less than m-d and since it is a normal (symetric) ditribution thus 16 percent is greater than m+d.suff

My answer is thus D

Last edited by yezz on 28 Sep 2006, 14:57, edited 1 time in total.
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GMATPREP-DS-Batteries [#permalink] New post 28 Sep 2006, 14:55
I thot it was C too but GMATPREP slapped me and was like NOOOOO its not C :cry:
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 [#permalink] New post 29 Sep 2006, 00:40
Some knowledge transfer from yezz again,

Thaks a lot yezz

Keep rocking.............
_________________

Averages Accelerated:Guide to solve Averages Quickly(with 10 practice problems)


Last edited by cicerone on 25 Sep 2008, 00:13, edited 1 time in total.
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 [#permalink] New post 29 Sep 2006, 06:24
Yep, the key point here is "symmetric about the mean" = normal distribution.

(D)
  [#permalink] 29 Sep 2006, 06:24
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