A bird is sitting on a train A moving at a speed of 40 kmph. It sees another train B at a Distance of 200 ms with Speed of 60 kmph coming from the opposite direction on the same rail track. It flies with an average speed of 10kmph and sits on another train. Again immediately it flies back to the first train and again to the second train and so on. It does so before the two trains crash. What is the total distance travelled by the bird?

Solution:

Two trains are moving in opposite direction with Speed 40 kmph and 60 kmph. Make train B stationary and then take the Speed of train A relative to the train B. So relative Speed of train A = 40 + 60 = 100 kmph. Before it crashes into the stationary train B it has to travel a Distance of 200 m which it will do in (0.2/100 ) hours. During this duration, the average Speed of bird has been 10 kmph i.e. before the crash total Distance covered by the bird = 0.2 * 10/100 km = 20 meters.
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