Crossett Truck Rental has a large fleet of rental trucks. At times many of the t

Question

Crossett Truck Rental has a large fleet of rental trucks. At times many of the trucks need substantial repairs. Mr. Crossett has requested a study of the repair costs. A random Sample of 64 trucks is selected. The mean annual repair cost is $1,200 with a standard deviation of $280.

a. Estimate the mean annual repair cost for all rental trucks: $1,200

b. Develop the 95% confidence interval for the population mean: 1131.4 to 1268.6

c. Suppose Crossett's fleet consists of 500 trucks. Develop the 95% confidencence interval for the population mean.

I am in need of help with part c (in bold). I keep getting the answer 1175.5 to 1224.5 but that isn't the answer. A break down of how to find this on a ti84/83 calculator is what I am looking for. Thanks.

Explanation / Answer

Part-c

Standard error of mean=280/sqrt(500)=12.522

Critical t=1.9983

95% CI= 1200-1.9983*12.522, 1200+1.9983*12.522

=(1174.977      1225.023)

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