Two trains are 150 kilometers apart and are heading toward each other along a single track. The first train is going 90 kilometers an hour. The second train is going 60 kilometers an hour. Flying at a constant speed of 110 kilometers an hour, a bird takes off from the head of the first train, flies to the head of the second, immediately turns and flies back to the head of the first, immediately turns and flies back to the head of the second, and keeps going like this until the two trains crash. You may assume that the turns that the bird makes are instantaneous.
How far does the bird fly?
The two trains take 150/(90+60) = 1 hour to smash together.
The bird flies 110 km in that time.