At 600 am two planes started from the same airport and flew

At 6:00 a.m. two planes started from the same airport and flew west. One plane averaged 520 miles per hour, and the other plane averaged 600 miles per hour. At what time were the planes 280 miles apart?

600T-520T=280
80T=280
T=280/80
T=3.5 HOURS AFTER THEY LEAVE THEY WILL BE 280 MILES APART.
6:00+3:30=9:30 am. ANS.