Question

A man walks at the speed 4km/h. A train coming from behind him at 22km/h passes by him in 30s. Find the length of the train

Answer 1

Let the person's car be Car A. When he first sees the car(Car B) behind him, that car is 400m(due to the visibility being 400m) behind him. Then, the car draws level with him, Car B has travelled 400m relative to Car A. Then when it overtakes and finally disappears, it has travelled another 400m(so that is not visible again). So the total distance travelled by Car B relative to Car A is 400m +400m = 800m = 0.8 Km. This distance is covered in 3 minutes or 1/20 hours. So in 1 hour, Car B would have travelled 0.8*20 = 16 Km. Hence the speed of Car B is 16 Km/h relative Car A. Now since Car A itself has a speed of 4 Km/h, the speed of Car B comes out to be (16+4) ie 20 Km/h.