The super fly
Two trains are on the same road, 100 km apart. They begin to move towards each other at a speed of 50km / h. At the same moment, a super-brand leaves the first locomotive of one of the trains and flies at 100 km / h to the locomotive of the other train. It just arrives, turns around and returns to the first locomotive, and so comes and goes from one locomotive to the other until the two trains collide and so die in the crash. How far has the super fly traveled?