another DS

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another DS

by superrrom » Mon Sep 01, 2008 7:05 am
I'm confused how the answer could be D.
Thanks
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by parallel_chase » Mon Sep 01, 2008 8:16 am
In such questions always remember mean is the mid point which means 50% values is greater than the mean and 50% are below the mean

Mean = m

SD=d

Always remember the 68-95-99.8 rule of statistics

Here is a link which will help you understand this rule
https://en.wikipedia.org/wiki/68-95-99.7_rule

Mean + 1stSD = m+d - 34%
Mean + 2ndSD = m+2d - 14%
Mean + 3rdSD = m+3d - 2%

similarly for below the mean

Mean - 1stSD = m-d - 34%
Mean - 2ndSD = m-2d - 14%
Mean - 3rdSD = m-3d - 2%

Statement I

68% of the values lies within m+d and m-d

This mean 34% of values lies within the 1st SD above the mean and 34% lies within 1st SD below the mean.

We have to find the percentage greater then 1st SD above the mean

50%-34% = 16%

Therefore sufficient.

Statement II

16% of the values are less than m-d

Therefore 16% of the values would be more than m+d.

Therefore sufficient.

Hence D is the answer.

Let me know if you have any doubts.

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by superrrom » Mon Sep 01, 2008 9:06 am
wow that's great to know. Thanks

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by Ian Stewart » Mon Sep 01, 2008 9:35 am
parallel_chase wrote:In such questions always remember mean is the mid point which means 50% values is greater than the mean and 50% are below the mean
This is true in this question, because you know the distribution is symmetric. When a distribution is symmetric, the median and the mean are equal. However, in many sets you'll encounter on the GMAT, the median and the mean will be different.
parallel_chase wrote:
Always remember the 68-95-99.8 rule of statistics
This rule only applies to data that is 'normally distributed', something which you can never assume to be the case on the GMAT (and you don't need to know what 'normally distributed' means, either). The 68-95-99.7 rule is not true for arbitrary distributions of data, nor is it tested on the GMAT. You don't need to know anything about normal distributions to solve the above problem, and you don't need the 68-95-99.7 rule; you only need to know what is meant by 'symmetric distribution'.
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