Answer: the second plane must travel 800 mph.
Step-by-step explanation:
Let x represent the speed of the second plane.
By the time both planes will meet, they would have covered the same distance.
Distance = speed × time
The first plane travels at 400mph.
Since the second plane caught up with the first plane in 2 hours, then the distance covered by the first plane is
400 × 2 = 800 miles
The second plane leaves an hour after the first plane. This means that the time spent by the second plane before it caught up with the first plane is 1 hour
Distance covered by the second plane is x × 1 = x
Since the distance covered is the same, then
x = 800 mph
