NYC5648 wrote:
A train passes a man standing on a platform in 9 sec. and passes the platform in 12 sec. completely. If the platform is 50 meters long, how long is the train?
The offical answer is: 150m
Can somebody help me please?
Many thanks!!!
Passing a man standing means the train travels a distance equal to its length. If we denote by L the train's length, it means the train travels the distance L in 9 seconds.
Passing the platform completely means the train travels a total distance equal to the length of the platform plus the length of itself, which is 50+L. We can deduce that the train travels 50m in 3 seconds, so in 9 seconds, it travels 3*50 =150m.
Therefore, the length of the train is L = 150m.
Try to visualize the two situations in order to understand in each case what is the distance traveled by the train.
When passing the man, timer starts when the man and the beginning of the train are aligned, and timer stops when the end of the train and the man are aligned. So, the distance traveled by the train is its length L.
When passing the platform, timer starts when the beginning of the train and one end of the platform are aligned and the timer stops when the end of the train and the other end of the platform are aligned. In this case, the train traveled 50m (the length of the platform) + its own length L.