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Forum index » Science and Technology » Math » Undergraduate
Standard Deviation Question.
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Matt
science forum beginner


Joined: 24 Dec 2005
Posts: 25

PostPosted: Sun May 21, 2006 4:15 pm    Post subject: Standard Deviation Question. Reply with quote
A die is rolled 360 times. Find the standard deviation for the number
of 3s that will be rolled.

a. 60
b. 50
c. 7.1
d. 5.9

I think it is 7.1, but not sure.
Thanks!

Best wishes,

Matt
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Brian M. Scott
science forum Guru


Joined: 10 May 2005
Posts: 332

PostPosted: Sun May 21, 2006 8:22 pm    Post subject: Re: Standard Deviation Question. Reply with quote
On 21 May 2006 09:15:38 -0700, Matt <matthewamack@yahoo.com>
wrote in
<news:1148228138.138746.87050@38g2000cwa.googlegroups.com>
in alt.math.undergrad:

Quote:
A die is rolled 360 times. Find the standard deviation for the number
of 3s that will be rolled.

a. 60
b. 50
c. 7.1
d. 5.9

I think it is 7.1, but not sure.

It is indeed 7.1 (or 7.07 to two decimal places). In effect
you're drawing 360 times (with replacement) from a box
containing a 1 and five 0s. The std. dev. s of those six
numbers is sqrt(5)/6, and the std. dev. of the sum of the
360 draws (= std. dev. of the number of 3s rolled) is
s*sqrt(360).

Brian
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