Explanation:

Given that:
Speed of the train = (72 x 5/18) m/sec.
20 m/sec.
Let the length of the train be x metres.
Then, x + 120 /20 = 60  
⇒ x + 120 = 1200  
⇒  x = 1080.
So, time taken by the train to pass an electric perch
= (1080 x 1/20)sec. = 54 sec.