A machine manufactures parts whose diameters vary according to the Normal distribution with mean ? = 40.150 millimeters (mm) and standard deviation ? = 0.003 mm. An inspector measures a random sample of 4 parts. The probability that the average diameter of these 4 parts is less than 40.148 mm is about Solution mean =40.15 SD =sd/sqrt(n) = 0.003/sqrt(4) = 0.0015 Thus : z = (40.148-40.150)/0.0015 = -1.33 P(Z) =0.09176.