720 mi. in 4.5 hrs is 160 mph.
720 mi. in 6 hrs is 120 mph.
Now flying with a tailwind the wind speed is added to the airspeed. Flying with a headwind it's subtracted from airspeed.
So find the difference between 120 and 160, that's 40, and divide that in half, and that's your wind speed, 20 mph. So the airspeed of the airplane would be 140 mph.
