Answer:
We can define speed as:
Speed = distance/time.
(time says 1 of, so i will use 1/3 of an hour, the math will work for any number, you can just change it)
Here the distance is 420 miles and the time is 1/3 hour, so the speed is:
S = 420mi/(1/3)h = 420mi/(1/3)h or 1260 mph.
Then, in a complete hour the airplane will travel other 1260 miles