To pass a physical fitness test, Marcella must run 2500 m at an average speed of 3.85 m/s. She runs the first 750 m at an average of 4.15 m/s. (a) How much time does she have to run the last 1750 m? (b) What should be her average speed over the last 1750 m in order to finish with an overall average speed of 3.85 m/s?