A man crosses a stationary train in 15 minutes. The same train crosses a man in 50 seconds. What is the respective ratio between the speed of the train and the man ?

When a man crosses a stationary train, his distance covered equals the train's length. When the train crosses a stationary man, the train covers its own length. Both scenarios give train length = speed_man × 15 min = speed_train × 50 sec. This gives speed_train/speed_man = (15 × 60)/50 = 18, BUT this assumes man's speed is constant across both scenarios, which is not necessarily true. Without this assumption, the ratio is undetermined.