Let point O be the airport
Flight B moves towards the north at a speed of 600 km/hr
Fight A moves towards the west at a speed of 800 km/hr
Distance OA=800×1=800 km
Distance OB=600×1=600 km
Distance between the flight after 1 hour = AB
Applying pythagoras theorem in triangle AOB
AB2=OA2+OB2
AB2=(800)2+(600)2
AB2=6400+3600
AB2=10000
AB =100 km