Control charts for x and R are maintained on the certain dimensions of manufactured part, measured in mm. The subgroup size is 4. The values x and R are calculated for each and every subgroup. After 20 subgroups, the mean is 412.83 and average range is 3.39. Determine the values of the plus or minus 3 standard deviation limits for x and R charts and determine the value of the standard deviation on the assumption that the procedure is in statistical control.