Mathematics, 01.04.2020 21:41 reeeg38

A random sample of the costs of repair jobs at a large muffler repair shop produces a mean of $127.95. and a standard deviation of $24.03. If the size of this sample is 40, which of the following is an approximate 90 percent confidence interval for the average cost of a repair at this repair shop

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Answer from: ttangelique

$127.95-1.685\frac{24.03}{\sqrt{40}}=121.55$

$127.95 +1.685\frac{24.03}{\sqrt{40}}=134.35$

So on this case the 90% confidence interval would be given by (121.55;134.35)

Step-by-step explanation:

Previous concepts

A confidence interval is "a range of values that’s likely to include a population value with a certain degree of confidence. It is often expressed a % whereby a population means lies between an upper and lower interval".

The margin of error is the range of values below and above the sample statistic in a confidence interval.

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".

$\bar X=127.95$ represent the sample mean for the sample

$\mu$ population mean (variable of interest)

s=24.03 represent the sample standard deviation

n represent the sample size

Solution to the problem

The confidence interval for the mean is given by the following formula:

$\bar X \pm t_{\alpha/2}\frac{s}{\sqrt{n}}$ (1)

In order to calculate the critical value $t_{\alpha/2}$ we need to find first the degrees of freedom, given by:

df=n-1=40-1=39

Since the Confidence is 0.90 or 90%, the value of $\alpha=0.1$ and $\alpha/2 =0.05$ , and we can use excel, a calculator or a table to find the critical value. The excel command would be: "=-T.INV(0.05,39)".And we see that $t_{\alpha/2}=1.685$

Now we have everything in order to replace into formula (1):

$127.95-1.685\frac{24.03}{\sqrt{40}}=121.55$

$127.95 +1.685\frac{24.03}{\sqrt{40}}=134.35$

So on this case the 90% confidence interval would be given by (121.55;134.35)

Answer from: Quest

walter needs to make at least 5 more cookies but no more than 40. 5 ≥ x ≥ 40;

step-by-step explanation:

walter has to make a minimum of 20 cookies. he already has 15 cookies made, so he must make 5 more to meet the minimum.

the maximum amount of cookies he can make is 55. he already has 15, so he has to make no more than 40 more cookies to meet the maximum.

Answer from: Quest

Yes you do, so £1973.09 - £1473.50 = £499.59 so tim overpaid £499.59 .hope this ; )

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