Walking home one day, you take a short cut along the train tracks. The tracks cross a narrow bridge over a deep gorge. At the point you are 3/8 of the way across the bridge, you hear the train whistle somewhere behind you. You charge across the bridge, and jump off the track as the train is about to run you down. As it happens, if you had gone the other way, you would have reached safety just before being run over as well. If you can run ten miles per hour, how fast is the train moving?
Compare your speed to the train's when
getting over the bridge...
The train is moving at 40 miles per hour.
if there was a dog going towards the train while you go away from it as the whistle blows, he will reach his point when
you are 3/4 (6/8) of the way on the bridge, the train will have just reached it. So for the train to cross the bridge at a rate four times yours it will have to be moving 4 times as fast.