Rockwell hardness of pins of a certain type is known to have a mean value of SD and a standard deviation of 1.5. {Round your answers to four decimal places.) I USE SALT {a} [f the distribution is normal, what is the probability that the sample mean hardness for a random sample of 11 pins is at least 51? E {b} What is the {approximate} probability that the sample mean hardness for a random sample of 42 pins is at least 51? E