SPEED TIME DISTANCE PLANE 6r 1200/(6r) 1200 TRAIN r 1200/r 1200 DIFFERENCE 10
If the plane travels six times as fast as the train, the train spends six times as long time as the plane: 6x hours for the train and x hours for the plane. The difference is 10 hours: 6x - x = 10 hours, according to the condition. Hence, 5x = 10 and x == 2 hours. Having 2 hours for the plane, you momentarily determine its speed: = 600 km/hours, exactly as in your answer.