17. a 120 m long train is moving in a direction with speed 20 m/s. a train b moving with 30m/s in the opposite direction and 130 m long crosses the first train in a timea) 6 s(b) 36 sc) 38 sd) 5 s

Answer is D.

Speed:
Use relative speed to simplify the situation. Since the trains are moving in opposite directions, you can add the speeds and pretend the first train is stationary (moving at 0m/s) and the second train is moving at 50m/s.

Distance:
The front of the second train needs to travel 120m to get from the front to the back of the first train. When the front of the second train is at the back of the first train, the back of the second train is still 10m in front of the first train. The back therefore has to travel 130m to clear the first train. The total distance over which the trains are overlapping in this scenario is therefore 120 + 130 = 250m.

You have speed and you have distance so now just calculate time:

v = d / t
50 = 250 / t
t = 5s

If an object is moving or in motion