13.278. A passenger on a train knows that the speed of this train on this section of the track is 40 km/h. As soon as a passing train started to go by the window, the passenger started a stopwatch and noticed that the passing train took $3 \mathrm{s}$ to pass by the window. Determine the speed of the passing train, given that its length is 75 m.
Solution.

Let $x$ be the speed of the oncoming train.

According to the condition, $(40+x) \cdot \frac{3}{3600}=75 \cdot 10^{-3}$, from which $x=50($ km $/ h)$.

Answer: $50 \mathrm{km} /$ h.