To estimate the time required to provide a given laboratory

To estimate the time required to provide a given laboratory procedure, suppose that we measured the amount of time required when the service was provided on 60 occasions.  Based on this sample we obtained a mean of 20.32 minutes and a standard deviation of 3.82 minutes.  What can we say with a probability of 0.95 about the size of the error when we use 20.32 minutes as the estimate of the true average time required to provide the procedure?