Martin Inc. has decided to use an R$-$Chart to monitor the changes in the variability of their $72\mathrm{.}00$

pound copper wires. The quality control manager randomly samples $9$

copper wires and measures the weight of the sample $($in pounds$)$ at $16$

successive time periods.

Table

Control Chart

Step $1$ of $7$: What is the Center Line of the control chart? Round your answer to three decimal places.