Fifteen subgroups of five parts each are collected (five consecutive parts every hour for 15 hours). a critical to quality dimension is measured and the average and range are calculated for each of the fifteen subgroups. the grand average (average of the 15 averages) is calculated to be 18.37 mm and the average range (average of the 15 ranges) is calculated to be 0.113 mm. what is the lower control limit on the average chart?