[填空题]
An airplane travels 310xrn rc06/*ex txgm:b60 fowce*8cm 9b4 km at a speed of 790 km/h , and then encounters a tailwind that boosts its speed to 990 k efmo 4cbc98w*m/h for the next 2800 km.
What was the total time for the trip? h
What was the average speed of the plane for this trip? [Hint: Think carefully before using Eq. 2–11d.] km/h