$6\mathrm{.}23$ The manufacturer wishes to set up a control chart at the final inspection station for a gas water heater. Defects in workmanship and visual quality features are checked in this inspection. For the past $22$ working days, $176$ water heaters were inspected and a total of $924$ nonconformities reported. a$.$ What type of control chart would you recom$-$ mend here and how would you use it$?$ b$.$ Using two water heaters as the inspection unit, calculate the center line and control limits that are consistent with the past $22$ days of inspection data.