Here is relatively simple one Two cars are 120 miles from each other on a straight road. They drive towards each other at 30 mph (relative to the road). At the instant they set off, a bird leaves one car, flying towards the other at 60 mph. When it reaches the second car it turns back to the first car, and continues flying back-and-forth till the cars pass each other. When the cars pass, how far has the bird flown? (Total distance, not 'how far from starting point.)