Mattel Corporation produces remote-controlled car that needs three AA batteries. The mean life of these batteries in this product is 38 hours. The distribution of the battery lives closely follows the normal probability distribution with a standard deviation of 5.9 hours. As a part of its testing program, Sony tests samples of 16 batteries.
a. What proportion of the samples will have a mean useful life of more than 39 hours?
b. What proportion of the sample will have a mean useful life greater than 36.5 hours?