42credit card holders are selected at random.For each, their current credit card balance is recorded.The average for these 42 people is= $600.Assume that the current balance of all credit card holders follows a normal distribution with unknown mean,and that a 85.3% confidence interval foris found to be $600 34.6.Find the standard deviationof the population?

Please also explain how to find z score from the a/2 = 0.07350. I am not understanding how to find the z score too.