3. Calculate the speed and length of the train that passes a stationary observer in 8 seconds, and passes a platform 480 meters long in 12 seconds.
Solution. Let $v$ be the speed of the train expressed in $\mathrm{m} / \mathrm{s}$, and $s$ be its length in meters. Then from the condition of the problem we have:

$$
v=\frac{s}{8} \text { and } v=\frac{s+480}{32} \text {. }
$$

From here we get $s=160 \mathrm{~m}$ and $v=20 \mathrm{~m} / \mathrm{s}(=72 \mathrm{~km} / \mathrm{h}$ ).