Confidence interval for single proportion.

1. The independent Department Store wants to determine the proportion of their charge accounts having unpaid balance of $500.00 or more. A sample of 250 accounts revealed that 100 of them had unpaid balance of $500 or over. What is the 99 percent confidence interval for the population proportion?

2. The manager of the A & B Supermarket wants to estimate the mean time a customer spends in the store. A 95 percent confidence level must be used. The standard deviation of population based on a pilot survey is estimated to be 3.0 minutes. The manager requires the estimate to be within 1.0 minutes of the population value. What sample size is needed?