6. Suppose that customers at a mall are surveyed about their total purchases made on a given shopping day. In a sample of 320 totals, 18 were for more than $100.

a) Calculate a 95% confidence interval for proportion of all customers whose total purchases are for more than $100.

b) Interpret your interval.

c) What type of interval did you use for part (a) (z-interval, t-interval, 1-prop-z-interval). Why?

d) What is the margin of error associated with your estimate?