Flight Incheon International Airport departs from Seoul Incheon International Airport at 18:45 and arrives at Da Nang Da Nang International Airport at 21:45.
The flighttime is about 5h 00m. The distance travelled ("as the bird flies"): 2275 Km. So the average speed is roughly 455 Km.
There is an 2 hour time difference between Seoul and Da Nang