At noon a small plane left an airport and flew due east at 200 km/h. Two hours later another plane left the same airport and flew due south at 400 km/h. At what time will the planes be exactly 800 km apart from one another?
Let x be the time in hours taken for the planes be 800 km apart.
Since first plane has left towards east and the second towards south, they moved in perpendicular directions and we can conveniently apply Pythagoras Theorem.