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Gmat Prep Please help - Mean and Standard Deviation

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by dubeystuts » Sat Jan 17, 2009 11:06 am
9.4, 9.9, 9.9, 9.9, 10, 10.2, 10.2, 10.5

The mean and the standard deviation of numbers are 10 and .3 respectively. What percent of the 8 numbers are within 1 standard deviation of mean?

1) 90%
2) 85%
3) 80%
4) 75%
5) 70%

I actually don't understand what the question is asking. Can someone please explain and the best approach for this question? Many Thanks.
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by DanaJ » Sat Jan 17, 2009 11:13 am
The question refers to the percentage of numbers that are within one std deviation (0.3, in this case) from the average (10, in this case). That "translates" to which numbers are between 10 - 0.3 and 10 + 0.3, that is 9.7 and 10.3. You have 6 out of 8 numbers of your series that fulfill that requirement (you have 9.9 three times, 10 once and 10.2 twice), which is a percentage of 75%.

Answer D.
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by dubeystuts » Sat Jan 17, 2009 11:36 am
Thank You very much for your promt reply and kind help! :D
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