You are just walking over a bridge. You have 3/8 of the bridge behind you, 5/8 ahead. Suddenly you hear a train approaching from behind (the 3/8 end), but it´s not yet on the bridge.
You know that you can turn around and run the shorter distance towards the train, or keep direction and run the longer distance. Either way, you would make it off the bridge just in time.
You run at 10 kph (or whatever velocity unit you prefer). How fast is the train?
If you need a calculator, you are overthinking this.