[填空题]
An airplane travels 3100 km at a speed of 790 km/h , and then *cr y 8ag9:( xz8rcmudeypjo tdu(q0 l3ylni f3 x-*ba2,t5j-k ncounters a tailwi3l qy do p*2j05(-injab 3k tuxl-,tfynd that boosts its speed to 990 km/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