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Gmat Prep - Please help

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by dubeystuts » Sat Jan 31, 2009 4:31 am
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

Can someone please explain? Many Thanks.
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Source: — Data Sufficiency |
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by DanaJ » Sat Jan 31, 2009 4:42 am
Since the distribution is symmetric about the mean, each statement by itself is sufficient.
1. tells us that 68% of batteries are within 1 std dev from the mean, which in turn gives us 100 - 68 = 32% more than a std dev away from the mean. Since the distribution is symmetric, each "slice" is 16% or 32/2: 16% of batteries are more than one std dev from the mean on each side.
2. since each sides are equal, that means that the other side will also be equal to 16%.

Answer D.
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