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?

“20 miles per hour”

This guess is incorrect because it suggests the train is moving at only twice the speed of the person running, which would not allow it to reach the bridge in time to hit the person when he is only 1/4 of the way across.

“30 miles per hour”

This guess also underestimates the speed of the train. At 30 miles per hour, the train would still not be fast enough to reach the edge of the bridge by the time the person runs the remaining distance, given the calculations based on their relative speeds.

“50 miles per hour”

While this guess is faster than the correct answer, it does not fit the scenario described in the riddle. The train's speed must be precisely four times the runner's speed, and 50 miles per hour exceeds that requirement.