There are numerous CEOs in the United States. A political scientist wants to estimate, with 99.7 % confidence and within  what the average cost for their support staff is each year. Based on...

There are numerous CEOs in the United States. A political scientist wants to estimate, with 99.7 % confidence and within
 what the average cost for their support staff is each year. Based on historical records the population standard deviation seems to be only. How many CEOs should be interviewed about their support staff?