Explanation:

First person speed = 2*(5/18) = 5/9 m/sec
Second person speed = 4*(5/18) = 10/9 m/sec

Let the length of train is x metre and speed is y m/sec

x /_{y-5/9 = 9}

9y - 5 = x

9y - x = 5 .............i

x /_y-10/9 = 10

90y - 9x = 100 ......ii

from i and ii we get,

x = 50

So length of train is 50 metre

Hence, option 2 is the correct answer.