SOLUTION: Ocho Rios Autoworks, a chain of automotive tune-up shops, advertises that its personnel can change oil, replace the oil filter, and lubricate any standard automobile in 15minutes,

Question 1192845: Ocho Rios Autoworks, a chain of automotive tune-up shops, advertises that its personnel can change oil, replace the oil filter, and lubricate any standard automobile in 15minutes, on the average. The Bureau of Standards received complaints from customers that service
takes considerably longer. To check the firm's claim, the Bureau had service done on 21 unmarked cars. The mean service time was 17 minutes, and the standard deviation of the sample was 1 minute. Use the 0.05 level of significance to check the reasonableness of the
claim made by Ocho Rios Autoworks.
Z = (X - U) / (SD / √n)
=17-15/1/sqrt(21)
= 9.165
I have never gotten a z value so large before, please advise if I am doing something wrong

Answer by Theo(13342) (Show Source): You can put this solution on YOUR website!
you did it right.
it is very possible to get z-score that are very high or very low.
this is one example of that.
it can get even higher than that.
assuming the average time was 30 minutes with a standard deviation of 1 minute and a sample size of 21, then the z-score could have been (30 - 15) / (1/sqrt(21)) = 68.7...
it all depends on what the actual wait time was versus what the claimed wait time was.

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