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running times & standard deviation

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by LevelOne » Fri Jul 03, 2009 7:29 am
sorry guys, if this has been posted before...

Q: 70, 75, 80, 85, 90, 105, 105, 130, 130 130

The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters. If the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 S.D. below the mean of the 10 running times?

A. one
B. two
C. three
D. four
E. five

thanks
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by ssmiles08 » Fri Jul 03, 2009 7:43 am
IMO B.

figure out the mean first. sum = 1000

avg = 1000/10 = 100

1SD = 100 - 22.4 ~ 78 (the decimal point shouldn't concern you b/c each point differs by 5 whole points and they are all whole numbers.)

so the only points that are below 78 are 75 and 70. (B)
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by LevelOne » Fri Jul 03, 2009 7:53 am
great thanks!
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