let

v1 = 200 kph the traveling speed of the first plane

v2 = the traveling speed of the second plane

t1 = the time the first plane traveled before the second plane caught up

t2 = the time the second plane traveled before it caught up with the first

another plane leaves an hour later traveling the same route

t2 = t1 - 1

3 hours after the first plane took off the second plane catches up with the first plane

t1 = 3 => t2 = 3-1 = 2

since the traveled the same distance and we know distance=speed*time

v1*t1 = v2*t2

200*3 = v2*2

v2*2 = 600

v2 = 600/2

v2 = 300 kph

the speed of the second plane is 300 kph.