Auto pistons at Wemming Chung's plant in Shanghai 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:                                                                 Day Mean Bold x overbar (mm) Range R (mm) 1 154.9 4.2 2 155.2 4.8 3 153.6 4.1 4 153.5 5.2 5 158.6 4.3 a) What is the value of x double overbar?