To pass a physical fitness test, Marcella must run 1000 m at an average speed of 4.00

Question:

To pass a physical fitness test, Marcella must run 1000 m at an average speed of 4.00 m/s. She runs the first 500 m at an average of 4.20 m/s.
(a) How much time does she have to run the last 500 m?
(b) What should be her average speed over the last 500 m in order to finish with an overall average speed of 4.00 m/s?