Question

a 120 m long train is moving with a speed 20m/s another train ( 130 m long) moving with 30m/s in opposite direction crosses the first train in a time?

Answer 1

The train approaches one another with 20 + 30m/s = 50m/s.

The first train has to cover the distance = 120 + 130 = 250

 Time is taken to cover the distance = 250/50

                                                             = 5s.

Hope this helps!

Answer 2

In relative motion when two bodies are approaching then their relative distance/velocity is sum of their individual distance/velocity

Time = Relative Distance / Relative velocity
= (120 m + 130 m) / (20 m/s + 30 m/s)
= 5 seconds

Both will cross with each other after 5 seconds