A quality analyst wants to construct a control chart for determining whether three machines, all producing the same product, are in control with regard to a particular quality variable. Accordingly, he sampled four units of output from each machine, with the following results:

Machine Measurements

#$117151517$

#$216251825$

#$323242322$

What is the estimate of the mean of the sampling distribution for the sample mean?

$19$

$16$

$20$

$23$

$21$