Power+, Inc produces AA batteries used in remote controlled toy cars. The mean life of these batteries follows the normal probability with a mean of 37 hours and a standard deviation of 5.5 hours.

As a part of its quality assurance program Power+, Inc. tests sample of 9 batteries

a) What proportion of the sample will have a mean useful life greater than 36.5 hours ?

b) what proportion of the sample will have a mean useful life between 36.5 and 38.5hrs? (Round z value to 2 decimal places and final 4