>For logic, try the following problem (it's amaizing how many people are not able to solve it):
>
>A train leaves Toronto at 7am. It is headed for Montreal and it runs at 80 km/h. Another train leaves Montreal for Toronto at 8am and it runs at 100 km/h. Both speeds are constant. The problem: which train is closer to Montreal when the two trains meet.
>

Before or after they collide? ;-)