From where did we get the highlighted (3) which represents Z?

Auto pistons at a plant in United States are produced in a forging process, and the diameter is a critical factor that must be controlled. From sample sizes of 10 pistons produced each day, the mean and the range of this diameter have been as follows: a) Draw Mean chart, considering that standard deviation is 1.75 . b) Draw the Mean-chart for this dimension, using the data observed in the table above. iolutions a) MEAN CHART (a) X=51569+1532+1536+1555+1566=155.16mm (b) R=54.2+4.6+4.1+5.0+4.5=4.48mm Cl=12