A Manager of a Cafeteria Wants to Estimate the Average $90 \%$

Short Answer

A manager of a cafeteria wants to estimate the average time customers wait before being served. A random sample of 49 customers has an average waiting time of 8.4 minutes with a standard deviation of 3.5 minutes.
-With $90 \%$ confidence, what can the manager conclude about the possible size of his error in using 8.4 minutes to estimate the true average waiting time?