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80 of the 100 elements of D are equal to x. The remaining 20 23 Sep 2006, 09:43
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80 of the 100 elements of D are equal to x. The remaining 20 elements are equal to y. What is |x-y| if the standard deviation of D is 400?

(A) 500 (B) 750 (C) 800 (D) 1000 (E) 1250



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80 of the 100 elements of D are equal to x. The remaining 20 23 Sep 2006, 09:50
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Kevin,
I seem to be getting 100??

Avg = 0.8x +0.2y

Variance = 16(x-y)^2

Std. Devn = sqrt(Var) = +4(x-y)

or 400 = +4(x-y)


|x-y| = 100??

Did I miss something??
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80 of the 100 elements of D are equal to x. The remaining 20 23 Sep 2006, 09:50
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kevincan
80 of the 100 elements of D are equal to x. The remaining 20 elements are equal to y. What is |x-y| if the standard deviation of D is 400?

(A) 500 (B) 750 (C) 800 (D) 1000 (E) 1250
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I am getting D

mean = (4x+y)/5

then calculating SD using the standard forumla.

does someone know a shortcut ?
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80 of the 100 elements of D are equal to x. The remaining 20 25 Sep 2006, 10:20
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Should be D...1000
calculated from the standard std deviation formula
400=sigma[sqrt([xi-x]/n)2]...
gives 0.4mod(y-x)=400...mod(y-x)=1000

what's OA?[/code]
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80 of the 100 elements of D are equal to x. The remaining 20 25 Sep 2006, 13:20
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Assume that x=0, so mean=y/5

sd^2= (80*y^2/25+ 20*16y^2/25)/100

so sd=2|y|/5

Thus |y-0|=1000 OA D
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80 of the 100 elements of D are equal to x. The remaining 20 21 Oct 2006, 08:31
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kevincan
Assume that x=0, so mean=y/5

sd^2= (80*y^2/25+ 20*16y^2/25)/100

so sd=2|y|/5

Thus |y-0|=1000 OA D
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Kevin: How did you get this

sd^2= (80*y^2/25+ 20*16y^2/25)/100 ?

Mean is y/5

So you muktiplied 80*(mean)^2 thats fine but for 20*16y^2/25 ?

Why did you take 16* something? Please explain
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80 of the 100 elements of D are equal to x. The remaining 20 21 Oct 2006, 08:34
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trivikram
kevincan
Assume that x=0, so mean=y/5

sd^2= (80*y^2/25+ 20*16y^2/25)/100

so sd=2|y|/5

Thus |y-0|=1000 OA D

Kevin: How did you get this

sd^2= (80*y^2/25+ 20*16y^2/25)/100 ?

Mean is y/5

So you muktiplied 80*(mean)^2 thats fine but for 20*16y^2/25 ?

Why did you take 16* something? Please explain
Show more



OK I got it now

SD^2 = Number of sample*(each value in sample - Mean)^2

Isnt it?



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