The shape of the distribution of the time required to get an oil change at a 20-minute
oil-change facility is unknown. However, records indicate that the mean time is
21.5 minutes, and the standard deviation is 4.3 minutes. Complete parts (a) through (c).
(a) To compute probabilities regarding the sample mean using the normal model, what
size sample would be required?
OA. The sample size needs to be greater than or equal to 30.
OB. The sample size needs to be less than or equal to 30.
OC. Any sample size could be used.
OD. The normal model cannot be used if the shape of the distribution is unknown.
(b) What is the probability that a random sample of n = 45 oil changes results in a sample
mean time less than 20 minutes?
The probability is approximately
(Round to four decimal places as needed.)
(c) Suppose the manager agrees to pay each employee a $50 bonus if they meet a
certain goal. On a typical Saturday, the oil-change facility will perform 45 oil changes
between 10 A.M. and 12 P.M. Treating this as a random sample, there would be a 10%
chance of the mean oil-change time being at or below what value? This will be the goal
established by the manager.
There is a 10% chance of being at or below a mean oil-change time of ☐ minutes.
(Round to one decimal place as needed.)