For a certain company, the relationship between revenue R (in millions) and number of employees n (in thousands) can be approximated by the following equation shown below. 4R =961nS In 2012, the company generated revenue of $39 billion, had 184,000 employees, and was adding employees at the rate of 10,000 per year. Find the rate of change of revenue with respect to time. About $ billion per year. (Round to the nearest billion as needed.)