To pass a physical fitness test, Marcella must run

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?