## 62. Math Puzzle $7 / 70$

Two trains pass each other in opposite directions. One travels at a speed of $36 \frac{\mathrm{km}}{\mathrm{h}}$, the other at $45 \frac{\mathrm{km}}{\mathrm{h}}$. A passenger in the second train noticed that the first train took 6 s to pass him.

How long was the train?
The speed at which the passenger in the second train moves relative to the moving first train is $45 \frac{\mathrm{km}}{\mathrm{h}}+36 \frac{\mathrm{km}}{\mathrm{h}}=81 \frac{\mathrm{km}}{\mathrm{h}}=22.5 \frac{\mathrm{m}}{\mathrm{s}}$.

Therefore, the length of the first train is $22.5 \frac{\mathrm{m}}{\mathrm{s}} \cdot 6 \mathrm{~s}=135 \mathrm{~m}$.