On two parallel railway tracks, two trains are moving towards each other at speeds of 72 km / h and 108 km / h.

On two parallel railway tracks, two trains are moving towards each other at speeds of 72 km / h and 108 km / h. The first train is 800 m long, and the second 200 m. How long does one train pass by another?

The total speed of the relative movement of trains:
V = V1 + V2, where V1 is the speed of the first train (V1 = 72 km / h = 72 * 1000/3600 = 20 m / s), V2 is the speed of the second train (V2 = 108 km / h = 108 * 1000/3600 = 30 m / s).
Distance traveled:
S = S1 + S2, where S1 is the length of the first train (S1 = 800 m), S2 is the length of the second train (S2 = 200 m).
Travel time:
t = S / V = (S1 + S2) / (V1 + V2) = (800 + 200) / (20 + 30) = 1000/50 = 20 s.
Answer: The movement time is 20 s.