Problem 1. A train is moving at a speed of $4 \mathrm{~m} / \mathrm{s}$. A bird is flying at a speed of $12 \mathrm{~m} / \mathrm{s}$. In 60 seconds, the bird flew from the end to the beginning of the train and back. How long is the train?
Solution. When the bird flies in the direction of the train's movement, its speed relative to the train is $12-4=8 \mathrm{~m} / \mathrm{s}$. When the bird flies in the opposite direction of the train's movement, its speed relative to the train is $12+4=16 \mathrm{~m} / \mathrm{s}$. Let x be the length of the train in meters. Then

$$
\begin{aligned}
& \frac{x}{8}+\frac{x}{16}=60 \\
& 3 x=960 \\
& x=320
\end{aligned}
$$

Thus, the train is 320m long.