longlast INc. produces car batteries. the mean life of these batteries is 60 months. the distribution of the battery life closely follows closely follows the normal distribution with a standard deviation of 8 months. as a part of its testing program, longest tests a samples of 25 batteries. what proportion of the samples will have a mean useful life of more than 56 months? what proportion of the samples will have a mean useful life of less than 56 months