A TGV departs from Paris at $150 \mathrm{~km} / \mathrm{h}$ heading towards Marseille, which is $800 \mathrm{~km}$ away. An intercity train departs from Marseille at $50 \mathrm{~km} / \mathrm{h}$ towards Paris at the same time. A swallow perched on the TGV takes off at this moment, at $200 \mathrm{~km} / \mathrm{h}$, heading towards Marseille. When it reaches the intercity train, it turns around, and when it meets the TGV again, it heads back the other way, etc. What distance will it have covered when the two trains meet?
The swallow flies at $200 \mathrm{~km} / \mathrm{h}$ for the 4 hours that elapse between the departure of the trains and the moment they cross. Therefore, it covers $800 \mathrm{~km}$.

Solution to exercise $3 \mathrm{Si}$, when ant $A$ and ant $B$ meet, if we swap $A$ and $B$, we get the same dynamics. We can thus assume that the ants cross each other instead of mutually repelling each other. Thus, each ant moves forward without turning around and eventually falls off.