$4.$ Six samples of five observations each have been taken of $80$kg concrete slabs produced by a machine, and the results are displayed below. $$ a$.$ Using factors from Table $10-3,$ determine the upper and lower control limits for sample mean and range, and decide if the process is in control. b$.$ A new sample results in the following slab weights: $81\mathrm{.}0,81\mathrm{.}0,80\mathrm{.}8,80\mathrm{.}6,$ and $80\mathrm{.}5.$ Use the control limits determined in part a to decide if the process is still in control.