A certain pay television company wishes to estimate the proportion of its customers who would purchase a pay television program guide. To do this, the company manager decides to select a random sample of customers and note the number who would purchase a program guide. Suppose that past experience in other areas suggests that about 30% of customers will purchase a program guide, How many customers should the company manager include in his sample if he wishes to estimate the true proportion who will purchase the program guide correct to within 5% (0.05) with a confidence level of 90%?

need working out, formula and answer