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# Urgent - Need Help for One standard Deveiation Question

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Intern
Joined: 29 Jun 2008
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Urgent - Need Help for One standard Deveiation Question [#permalink]  12 Jul 2009, 19:42
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The List consist of the times in seconds that took each of the schoolchildren to run a distance of 400 meters. If the standard deveiation 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 one standard deviation below the mean of the 10 running times?

1. One
2. Two
3. Three
4. Four
5. Five

Answer is two, but i dont know how to calcualte this one though it looks simple ... Pleae help me .. have exam in next 3 weeks.

Regards,
Puneet Pareek
Current Student
Joined: 03 Aug 2006
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Re: Urgent - Need Help for One standard Deveiation Question [#permalink]  12 Jul 2009, 20:33
Puneet,

Your question is missing the list of the 10 running times in seconds. Once you have that list this is how you would proceed.

1. Calculate the mean (avg) of the 10 numbers

$$\Rightarrow \frac{N1+N2+N3+...+N8+N9+N10}{10}=N_{avg}$$

2. Subtract the SD i.e. 22.4 from the avg

$$\Rightarrow N_{avg}-SD=N_{avg}-22.4$$

Now look for running times in the list that are less than this calculated value. The answer should be the number of values that satisfy the above criteria.
Intern
Joined: 29 Jun 2008
Posts: 15
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Kudos [?]: 2 [0], given: 0

Re: Urgent - Need Help for One standard Deveiation Question [#permalink]  14 Jul 2009, 18:27
Hi Nookway,

I got my answer... thanks a ton ... i calculated .. .answer is two only ...

Regards,
Puneet Pareek
Re: Urgent - Need Help for One standard Deveiation Question   [#permalink] 14 Jul 2009, 18:27
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