The passenger on the first train tracks the time of the oncoming train. Write down how long

The passenger on the first train tracks the time of the oncoming train. Write down how long the stopwatch will show if the speed of the first train is 36 km / h, and the speed of the second is 54 km / h, and the length of the second train is 0.25 km.

First, let’s calculate the total speed of the first and second oncoming trains. It is equal to their sum, since they move in different directions:
u = u1 + u2 = 36 km / h + 54 km / h = 90 km / h.

Let’s write down the formula for finding the speed:
u = s / t,

where s is the length of the oncoming train,
t is the time shown by the stopwatch.

From here we deduce the time shown by the passenger’s stopwatch:

t = s / u.

Let’s substitute the total speed and length of the train into it:
t = 0.25 / 90 = 1/360 h = 10 s.

Answer: 10 s.