In order to pass a physical fitness test, Marcella must run 1000 m at an average speed of 3.90 m/s. She runs the first 250 m at an average of 4.05 m/s. How much time does she have to run the last 750 m? What should be her average speed over the last 750 m in order to finish with an overall average speed of 4.20 m/s? Any help is appreciated, thanks!