Two trains travel on the same railway in opposite direction. The initial distance between the trains is 20 km. The speeds of the two trains are 108 km/h and 162 km/h. After how many seconds will the crash occur?

To find out when the two trains will crash, we need to determine how long it will take for them to cover the initial distance of 20 km when traveling towards each other.

Step 1: Convert the speeds from km/h to m/s for easier calculations.
- Train 1 speed: 108 km/h = (108 * 1000) m/3600 s ≈ 30 m/s
- Train 2 speed: 162 km/h = (162 * 1000) m/3600 s ≈ 45 m/s

Step 2: Calculate the relative speed by adding the speeds of the two trains.
- Relative speed = Speed of Train 1 + Speed of Train 2
- Relative speed = 30 m/s + 45 m/s = 75 m/s

Step 3: Divide the initial distance by the relative speed to find the time it takes for the two trains to crash.
- Time = Distance / Speed
- Time = 20 km / (75 m/s)

Finally, we need to convert the remaining kilometers to meters.
- 20 km = 20,000 m

Step 4: Calculate the time it takes for the two trains to crash.
- Time = 20,000 m / (75 m/s)
- Time ≈ 266.67 seconds

Therefore, it will take approximately 266.67 seconds for the two trains to crash.