# 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?

### 7 Answers

- AlanLv 72 months agoFavourite answer
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

- MyRankLv 62 months ago
1.28 = (x-130) / (10)

1.28 x 10 = (x-130)

12.8 = (x-130)

x = 130 + 12.8

Source(s): http://myrank.co.in/- ...Show all comments
so the correct answer is 134.53

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- Anonymous2 months ago
Post your work.

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- ideaquestLv 72 months ago
.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

- ...Show all comments
no you said 8 measurements

population standard deviation/sqrt(N) = 10/ sqrt(8)- Log in to reply to the answers

- Damon LyonLv 72 months ago
multiply both sides by 10

2.5 = x - 130

add 130 to both sides

132.5 = x

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Try solving

1.2816 = (x-130)/ (10 /sqrt(8) )

for x

since that is the correct equation.- Log in to reply to the answers

- Donald KLv 62 months ago
Remember the order of operations. Look for it in your textbook. I'm not doing your homework for you.

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thank you for ur help!!!!