## 245. Math Puzzle 10/85

Two trains pass each other while traveling in opposite directions. One at a speed of $36 \mathrm{~km} / \mathrm{h}$, the other at $45 \mathrm{~km} / \mathrm{h}$. A passenger in the second train noticed that it took 6 seconds for the first train to pass by him.

How long was the train that passed by him?
The speed at which the passenger in the second train moves relative to the moving first train is $45 \mathrm{~km} / \mathrm{h}+36 \mathrm{~km} / \mathrm{h}=81 \mathrm{~km} / \mathrm{h}=\frac{45 \mathrm{~m}}{2 \mathrm{~s}}$.

Therefore, the length of the first train $=\frac{45 \mathrm{~m}}{2 \mathrm{~s}} \cdot 6 \mathrm{~s}=135 \mathrm{~m}$.