A passenger train departs from $A$ to $B$ at 12 noon, traveling at a constant speed of $60 \mathrm{~km} /$ hour. Meanwhile, on the same track, a freight train travels from $B$ to $A$ at a speed of $40 \mathrm{~km} /$ hour. Both trains arrive at their destinations at the same time, and at this moment, they are three times as far apart as they were one hour before their meeting. When did the freight train depart from $B$?
Within an hour, the passenger train covers $60 \mathrm{~km}$, while the freight train covers 40. Thus, one hour before their meeting, they were $100 \mathrm{~km}$ apart, and their entire journey was $300 \mathrm{~km}$. The passenger train completes this in 5 hours, and the freight train in 7.5 hours, so the latter started $21 / 2$ hours earlier than the passenger train, which means it started at $9^{30}$.