A 1976 study of women smokers over 30 years of age who delivered premature babies

A 1976 study of women smokers over 30 years of age who delivered premature babies (fewer than 260 days of gestation) reported an infant survival percentage of 72%. 

Suppose researchers would like to estimate the current survival proportion. Using the survival rate from the 1976 study as a preliminary estimate of the current survival proportion, compute the minimum sample size required to estimate the current survival proportion to within .05 with 95% confidence.