Question: how to calculate sample size with unknown standard deviation?
Answer: za/2: Divide the confidence interval by two, and look that area up in the z-table: .95 / 2 = 0.475. ...  E (margin of error): Divide the given width by 2. 6% / 2. ...  : use the given percentage. 41% = 0.41. ...  : subtract. from 1.