The bird and the cars
Consider a road with two cars, at a distance of 100 kilometres, driving towards each other. The left car drives at a speed of forty kilometers per hour and the right car at a speed of sixty kilometers per hour. A bird starts at the same location as the right car and flies at a speed of 80 kilometers per hour. When it reaches the left car it turns its direction, and when it reaches the right car it turns its direction again to the opposite, etcetera.
The Question: What is the total distance that the bird has traveled at the moment that the two cars have reached each other?
If you have written down a whole page full of mathematical formulae, then you have probably been thinking in the wrong direction for this puzzle. The two cars will meet each other after one hour, hence the bird has been flying for one hour.
The bird has flown 80km when the cars meet.