alevel statistics variance question HELP URGENT Watch

anaveragetoad
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The lengths of 45 ropes used at an outdoor recreational center can be extended by 30% when stretched. the sum of squares of their stretched lengths is 0.0507km^2 and their natural lengths, x metres, are summarized by ∑(x-20)^2=1200. find the mean natural length of these 45 ropes.
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anaveragetoad
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hey helpppp
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mqb2766
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(Original post by anaveragetoad)
hey helpppp
You must have done an equation that relates variance to both the mean and the sum of squares?
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Physics Enemy
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(Original post by anaveragetoad)
The lengths of 45 ropes used at an outdoor recreational center can be extended by 30% when stretched. the sum of squares of their stretched lengths is 0.0507km^2 and their natural lengths, x metres, are summarized by ∑(x-20)^2=1200. find the mean natural length of these 45 ropes.
Sum (1.3x)^2 = 1.3^2 Sum x^2 = 50700 m^2
Sum x^2 = 50700/1.3^2 = 30000

Var(X) = Var(X - 20)
(Sum x^2)/n - u^2 = [Sum (x - 20)^2]/n - (u - 20)^2
Solve for u, you'll get an integer
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mqb2766
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Hints not solns pls
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