You wish to estimate, with 95% confidence, the population proportion of U.S. adults who think Congress is doing a good or excellent job. Your estimate must be accurate within 4% of the population proportion.

(a) No preliminary estimate is available. Find the minimum sample size needed.

(b) Find the minimum sample size needed, using a prior survey that found that 25% of U.S. adults think Congress is doing a good or excellent job. 

(c) Compare the results from parts (a) and (b).