The problem can be solved by considering the relative speed of the trains.
The speed of approach of the trains is 125 km/h on a 100 km track, so they will collide after a time 100/125 = 4/5 hour. Sidney flies at 100 km/h for 4/5 hour so he flies 80 km.
Finally, if Sidney were to fly exactly 100 km, the relative speed of the trains would be 100 km/h, provided Sidney was continuing to fly at 100 km/h. OR ..... there is another way - do you have any suggestions?
We received the correct solutions from: Stephen Smith(South Greenhoe Middle School).