Please Help! Brain teaser quiz for my mystery, magic and myth cl!!?

Two trains start toward each other at the same time on a single track from two stations which are 50 km apart. Just as the trains start out, a bird leaves the first train and flies directly to the other train, and as soon as the bird reaches the second train it starts back toward the first train. This goes on until the trains meet. If bother trains travel at 25 km/h and the bird travels at 100km/h, how many kilometres would the bird have flown before the trains meet?