A consumer group wants to estimate the mean electric bill for the month of July for singlefamily

Question:

A consumer group wants to estimate the mean electric bill for the month of July for singlefamily homes in a large city. Based on studies conducted in other cities, the standard deviation is assumed to be $25. The group wants to estimate, with 99% confidence, the mean bill for July to within ±$5.
a. What sample size is needed?
b. If 95% confidence is desired, how many homes need to be selected?