what is the value of f(n), when n is positive integer.
2. When a train passes by the rail, the passenger will hear a "clong" when it passes the joining part of the rail which is 10m long for each track. How many second will be needed for the passenger to count the number of time of the "clong" that equals to the exact speed of the train in km per hour?
更新1:
While riding on a train you can hear the click of the wheels passing over the joins in the rails. If each rail is 10m long, how many seconds must a passenger count clicks to equal the exact speed of the train in kilometres per hour? (original verison)