Eyestrain Is Thought to Be Associated with Different Types of Office

Eyestrain is thought to be associated with different types of office work. The Acme Temp-Help Company provides short-term employees to substitute for vacationing data entry workers. Previous research has shown that approximately 21% of full time data entry workers have eyestrain, and Acme would like to estimate the proportion, p, of their 6,000 part-time data entry employees that have eyestrain.
A) They would like to estimate p to within 0.05 with 95% confidence. If they accept the value of 0.21 for full-time data entry workers as a reasonable initial estimate of p, what sample size should they use for their study?
B) Suppose Acme believed that the working conditions of their temporary data entry personnel are different enough that they should not depend on the 0.21 as an initial estimate. In a few sentences, explain how your procedure for choosing a samples size would differ from your solution in part (a). (Do NOT recalculate a new estimate of the necessary sample size!)