A consulting company wants to estimate the proportion of Americans who own their house. What sample size should be obtained if the estimate is...

A consulting company wants to estimate the proportion of Americans who own their house.  What sample size should be obtained if the estimate is expected to be within 0.04 with 95% confidence if; a) the use an estimate of 0.675 from the Census Bureau: b) they do not use any prior estimates?  But in solving this problem, you are actually suing a form of "prior" estimate in the formula used.  In this case, what is your "actual" prior estimate?  Please explain.