How about this one......
you are walking along a single railway track, you come to a tunnel and start walking through it.
you get exactly a quarter of the way through when you hear a train coming behind you.
If you run back up the track (the way you came and towards the train), you escape just as the train reaches the tunnel entrance.
If you run down the rest of the tunnel (away from the train) you escape just as the train leaves the tunnel.
Assuming you run at a constant 12 mph, how fast is the train traveling?