With a tail wind a plane took 3 hours to travel 1440 km On t

With a tail wind, a plane took 3 hours to travel 1440 km. On the return trip against the same windm, the trip took an extra hour. Find the speed of the plane in still air?

Speed downwind = 1440/3 = 480 km/hr
Speed upwind = 1440/4 = 360 km/hr
The airspeed is the average, 420 km/hr