Power+, Inc. produces AA batteries used in remote controlled toy cars. The mean life of these batteries follows the normal probability distribution with a mean of 37 hours and a standard deviation of 5.5 hours. As a part of its quality assurance program. Power+, Inc test samples of 9 batteries?
a) What proportion of the sample will have a mean useful life of more than 38.5 hours? ( Round z value to 2 decimal places and final answer to 4 decimal places)