First, the train travels at 18 m/s for (2000/18) seconds.
Then it travels at 30 m/s for (2000/30) seconds.
Average speed = (total-distance)/(total-time)
Total distance = 4000 meters
Total time = (2000/18 + 2000/30) s
That's (10,000/90 + 6,000/90)
That's (16,000/90) or (1600/9) sec
Average speed = (4000•9/1600) m/s
Average speed = 22.5 m/s
That speed is LESS THAN 25 m/s .