Explanation:

Let the length of the train be x meters and its speed by y m/sec.
Then, x/y = 9
⇒   x = 9y.
Now, x + 420 /30 = y  
⇔   30y = x + 420  
⇒ 30y = 9y + 420.
⇒  21y = 420 
⇒  y = 20.
Speed  =  20 m/sec  = (20 x 18/5) = 72km/hr