Anonymous
Anonymous asked in Science & MathematicsMathematics · 2 months ago

# need tutor with this question ?

I got 142.8 for first time and then got 132.5 for second time, i use .25=(x-130)/10, what's the wrong part for this question? Relevance

I use .25=(x-130)/10

You shouldn't use this .

First , since this is question about the mean of eight measurements

The standard deviation = population standard deviation/sqrt(N)

so standard deviation = 10/sqrt(8)  not just 10

second P(z>Z) =0.1

means P(z<Z) = 0.9

if you reverse lookup , P(z<approx. 1.2815) = 0.9

so your starting equation should be

1.2815  = (x-130) /  (10/sqrt(8))

or if your class allows you to use the closest Z value

1.28 = (x-130)/ (10/sqrt(8))

start here and see if you come with my answer.

P(X_bar > L)  = 1- P(X<L)

P(X_Bar>L ) = 0.1

0.1 = 1 -P(X_bar<L)

P(X<L)  = 1-0.1 =0.9

P(z<Z) = 0.9

P(z< 1.28) =  0.89973 .

P(z<  1.29      )  = 0.90147

If you interpolate, you get

P(z< 1.2815)  =  0.90

Z = (L-mean)/ (standard deviation/ sqrt(N))

1.2815 = (L-130)/ (10/sqrt(8))

1.2815*10/sqrt(8) = L -130

L = 130+ 1.2815*10/sqrt(8) =  134.5307867

L = 134.53  (if you interpolate)

if you take the closest value for Z = 1.28

L = 130 + 1.28*10/sqrt(8) =    134.5254834

which still rounds to

L = 134.53

• thank you for ur help!!!!

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• 1.28 = (x-130) / (10)

1.28 x 10 = (x-130)

12.8 = (x-130)

x = 130 + 12.8

• so the correct answer is 134.53

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• Anonymous
2 months ago

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• Anonymous
2 months ago

How did you get 142.8? You must have made a mistake.

• yes i totally make a big mistake when i did the second steps

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• .25 = (x - 130) / 10

multiply both sides by 100,

25 = 10 (x - 130)

25 = 10 x - 1300

10 x = 25 + 1300

10 x = 1325

x = 132.5

• no you said 8 measurements
population standard deviation/sqrt(N) = 10/ sqrt(8)

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• multiply both sides by 10

2.5 = x - 130

132.5 = x

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