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GMAT Prep: Standard Deviation

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by dnkcdnguy » Mon Feb 16, 2009 1:45 pm
Can someone please help out with this question. I'm still a little rusty when it comes to the SD questions.

Thanks
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by Brent@GMATPrepNow » Mon Feb 16, 2009 3:05 pm
First calculate the mean. The mean = 100
If the SD is 22.4, then one standard deviation is 22.4 (two standard deviations is 44.8 etc)
So, one standard deviation ABOVE the mean is 122.4 (100+22.4) and one standard deviation BELOW the mean is 77.6 (100-22.4)

So, the question is really asking, "How many running times are less than 77.6?"
We can see that 2 running times (70 and 75) are less than 77.6.

I hope that helps.
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by dnkcdnguy » Mon Feb 16, 2009 6:26 pm
Thanks Brent!

That does help and I definitely won't forget these types of SD questions anymore.

:)
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