42 credit 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 μ,and that a 85.3% confidence interval for μ is found to be $600 ± 34.6. Find the standard deviation σ of 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.